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)

_translations/fr.yml

@@ -78,7 +78,7 @@ fr:
     forums: "Forums"
     bitcointalk: "<a href=\"https://bitcointalk.org/index.php?board=13.0\">Forum BitcoinTalk</a>"
     reddit: "Communauté Bitcoin sur Reddit (anglais)"
-    stackexchange: "StackExchange (Q&R, anglais)"
+    stackexchange: "StackExchange (Q&amp;R, anglais)"
     irc: "Clavardage IRC"
     ircjoin: "Canaux IRC sur <a href=\"https://webchat.freenode.net/?channels=bitcoin&amp;uio=d4\">freenode</a>."
     chanbitcoin: "(Bitcoin en général, anglais)"