QA: Use HTML Proofer To Check Internal Links

Uses Ruby html-proofer to check the links.  This commit also fixes the
various problems it found, as well as dealing with some of its
non-problem complaints (it doesn't like anchor (a) tags without either
an href, name, or id).

Running HTML proofer takes about 12 minutes on my system (with up to two
threads), during which it prints no text. Travis CI times out after 10
minutes of nothing being written to stdout, so this commit also adds a
background process the Makefile to print a line every minute while make
runs.

Makefile

@@ -28,7 +28,7 @@ test: pre-build-tests post-build-tests
 valid: 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: travis-background-keepalive pre-build-tests build post-build-tests
 
 
 
@@ -49,9 +49,8 @@ pre-build-tests: pre-build-tests-fast
 
 ## All post-build tests, including those which might take multiple minutes
 post-build-tests: post-build-tests-fast \
-    check-for-broken-bitcoin-core-download-links
-	@ true ## SOMEDAY: use linkchecker to find broken links 
-	@      ## after this bug is fixed: https://github.com/wummel/linkchecker/issues/513
+    check-for-broken-bitcoin-core-download-links \
+    check-html-proofer
 
 ## All manual updates to content that should be run by a human. This
 ## will create or update files which should then be diffed and commited.
@@ -209,3 +208,9 @@ check-for-broken-bitcoin-core-download-links:
 	      curl -sI "$$url" ; \
 	    fi | grep -q '200 OK' || echo "Error: Could not retrieve $$url" ; \
 	  done | eval $(ERROR_ON_OUTPUT)
+
+check-html-proofer:
+	$S bundle exec ruby _contrib/bco-htmlproof
+
+travis-background-keepalive:
+	$S { while ps aux | grep -q '[m]ake' ; do echo "Ignore me: Travis CI keep alive" ; sleep 1m ; done ; } &