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/nl.yml

@@ -222,7 +222,7 @@ nl:
     devcommunitiesintro: "De volgende chatrooms en websites bespreken de ontwikkeling van Bitcoin. Lees alstublieft hun gedragsegels voordat u berichten plaatst."
     ircjoin: "<a href=\"https://webchat.freenode.net/?channels=bitcoin-dev\">IRC Kanaal #bitcoin-dev</a> op \"freenode\"."
     stackexchange: "<a href=\"https://bitcoin.stackexchange.com/\">Bitcoin StackExchange</a>"
-    bitcointalkdev: "<a href=\"https://bitcointalk.org/index.php?board=6.0\">BitcoinTalk Forum voor Ontwikkeling & Technische Bespreking</a>"
+    bitcointalkdev: "<a href=\"https://bitcointalk.org/index.php?board=6.0\">BitcoinTalk Forum voor Ontwikkeling &amp; Technische Bespreking</a>"
   download:
     title: "Download - Bitcoin"
     pagetitle: "Download Bitcoin Core"