QA: Check HTML Correctness & Fix Existing Errors

- _contrib/bco-htmlproof: check HTML for correctness; fail on any errors

- _contrib/bco-htmlproof: accept path for individual page to help debug
  page problems

- (Many files) Convert `&` in numerous elements to `&`

- _templates/download.html: use Liquid filter to automatically escape
  `&` in magnet links.  Also premptively tell HTML not to check the
  magnet link when checking external links (this check is not currently
  enabled)

- _releases/*: Escape `<parameter>` used in multiple Bitcoin Core
  release notes

- _templates/choose-you-wallet.html: change mSigna URL from
  .../coinvault&referer=bitcoin.org to .../coinvault?referer=bitcoin.org

- _templates/community.html: fix duplicate anchors by renaming one
  anchor

- _templates/events.html: move Javascript to separate file because it
  contains forbidden HTML close tags within the `<script></script>`
  tags.

- (Many files, mostly in _translations/) Fix many broken open tags or
  missing close tags.

- _translatios/zh_TW.yml & ko.yml: fix a total of three invalid
  characters (control characters)

_plugins/contributors.rb

@@ -60,7 +60,7 @@ module Jekyll
         x['contributions'] = c['contributions']
         # Set avatar_url when available
         if c.has_key?('avatar_url') and c['avatar_url'].is_a?(String) and /^https:\/\/avatars\.githubusercontent\.com\/u\/[0-9]{1,10}\?v=[0-9]{1,2}$/.match(c['avatar_url'])
-          x['avatar_url'] = c['avatar_url'] + '&size=16'
+          x['avatar_url'] = c['avatar_url'] + '&size=16'
         end
         # Set login when available
         if c.has_key?('login') and c['login'].is_a?(String) and /^[A-Za-z0-9\-]{1,150}$/.match(c['login'])