Upgrade to Jekyll 3.0

Gemfile:

  - Upgrade to Jekyll 3.x (3.0.1 tested).  This brings several new
    features I want to use, most notably *collections* which allows us
    to add blog-like collections. I've converted the `_releases` and
    `_alerts` pages into collections, although their plugins are
    maintained to handle the Download and Active Alert features.

  - Upgrade to latest Kramdown.

  - Lock Less at 2.4.0.  This prevents breaking our Less plugin.  Jekyll
    3.x provides native support for SCSS, so we may want to switch to
    that in time.

  - Lock HTML Proofer at 2.1.0.  The most recent version was taking
    forever to check our pages (I never actually got it to complete).
    I'll look into it when I get more time.

Makefile:

  - New `make clean` command.  Jekyll 3.x by default attempts to do
    incremental rebuilds.  The new `jekyll clean` command cleans up the
    metadata necessary for than so that a full build is performed, and
    this new `make clean` command is a wrapper around it so that we
    automatically do full rebuilds in the relevant cases.  Note: our
    plugins aren't fully compatible with the incremental rebuilds, but
    I'd like to fix that in the future.

  - Remove WEBrick hack to enable previewing with default URL paths (/
    instead of /index.html).

  - Filter out compliants from Rouge

README.md:

  - Now that Alerts (_alerts) are part of a collection, the file names
    are no longer parsed for dates, so instructions on adding the date
    to the YAML metadata have been added.

_alerts/*:

  - Now that alerts are part of a collection, the file names are no
    longer parsed to provide dates, so a `date:` field has been added to
    the YAML metadata.

_config.yml:

  - Some variables renamed per upgrade instructions.

  - Switched from old default syntax highlighter Pygments to new default
    Rouge.  I tried to use Rouge options to keep new output as similar
    to old output as possible to making diffing easy, but Rouge adds
    extra CSS class info.

  - Move `_alerts` and `_releases` into Jekyll 3.x "collections", which
    provide the organizational features we were using plugins to
    manange.  I haven't removed the old plugins because we still use
    some of their features (alerts.rb provides active issue and banner
    features; releases.rb provides info to Download page)

  - _layouts/* can no longer provide default global metadata; that is now
    provided in the new `defaults:` section in _config.yml.

_layouts/*:

  - Default metadata can no longer be provided in the layout files for
    collections, so I've removed it and left a message to see
    _config.yml.

_plugins/*:

  - Remove filter_for.rb. It's completely broken on Jekyll 3.x because
    of changes to Liquid which prevent adding new arguments to the
    inherited Liquid::For class. Existing uses of filter_for have been
    migrated to built-in for loops prefaced by sorts.

  - Remove remove-html-extension.rb: at it said in the comments, this
    was a temporary hack to get us to Jekyll 3.0.

_releases/*:

  - Rename all the files: prefix a v to the file name so the output html
    (e.g. v10.0.0.html) is the same as the source filename (e.g.
    v10.0.0.md).  This is necessary to migrate them to a Jekyll collection.

  - Remove %v from titles: we have to explicitly set the title, like we
    used to.  Again required for migration to collections.

_templates/events.html & en/rss/events.rss:

  - Sort events by date and then loop with regular for loop rather than
    filter_for

en/alerts.html & en/rss/alerts.rss:

  - Sort alerts by date and then loop with regular for loop rather than
    filter_for

en/bitcoin-core/index.md & en/version-history.html & en/rss/releases.rss:

  - Sort alerts by date and then loop with regular for loop rather than
    filter_for

Makefile

@@ -12,33 +12,34 @@ JEKYLL_LOG=._jekyll.log
 #######################
 
 ## `make` (no arguments): just build
-default: build
+default: clean build
 
 ## `make preview`: start the built-in Jekyll preview
-preview:
+preview: clean
 	$S bundle exec jekyll serve
 
 ## `make test`: don't build, but do run all tests
 test: pre-build-tests post-build-tests
 
 ## `make valid`: build and run fast tests
-valid: pre-build-tests-fast build post-build-tests-fast
+valid: clean pre-build-tests-fast build post-build-tests-fast
 
 ## `make all`: build and run all tests
-all: pre-build-tests build post-build-tests
+all: clean pre-build-tests build post-build-tests
 
 ## `make deployment`: for use on build server
-deployment: install-deps-deployment \
+deployment: clean install-deps-deployment \
     valid
 
 ## `make travis`: for use with Travis CI
-travis: travis-background-keepalive \
+travis: clean travis-background-keepalive \
     install-deps-development \
     all
 
 
 
 
+
 ## Install dependencies (development version)
 install-deps-development:
 	bundle install
@@ -95,6 +96,9 @@ manual-checks: manual-check-diff-sha256sums
 #################
 ERROR_ON_OUTPUT="sed '1s/^/ERROR:\n/' | if grep . ; then sed 1iERROR ; false ; else true ; fi"
 
+clean:
+	$S bundle exec jekyll clean
+
 ## Always build using the default locale so log messages can be grepped.
 ## This should not affect webpage output.
 build:
@@ -107,13 +111,9 @@ build:
 
 ## Jekyll annoyingly returns success even when it emits errors and
 ## exceptions, so we'll grep its output for error strings
-#
-## FIXME: temporarily ignoring errors from WEBrick because
-## _plugin/remove-html-extension does something hackish until we upgrade
-## to Jekyll 3.0.0
 check-for-build-errors:
 	$S egrep -i '(error|warn|exception)' $(JEKYLL_LOG) \
-	    | grep -vi webrick.*filehandler \
+	    | grep -vi 'rouge/lexers/shell.rb' \
 	    | eval $(ERROR_ON_OUTPUT)