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)

index.html

@@ -8,7 +8,7 @@ id: index
 title: Bitcoin
 ---
 <p class="mainsummary">{% translate listintro %}</p>
-<div class="mainvideo"><iframe src="//www.youtube.com/embed/Gc2en3nHxA4?rel=0&showinfo=0&wmode=opaque{% if page.lang != 'en' %}&cc_load_policy=1&hl={{ page.lang }}&cc_lang_pref={{ page.lang }}{% endif %}" frameborder="0" allowfullscreen></iframe></div>
+<div class="mainvideo"><iframe src="//www.youtube.com/embed/Gc2en3nHxA4?rel=0&amp;showinfo=0&amp;wmode=opaque{% if page.lang != 'en' %}&cc_load_policy=1&hl={{ page.lang }}&cc_lang_pref={{ page.lang }}{% endif %}" frameborder="0" allowfullscreen></iframe></div>
 <div class="mainlist">
   <div><div><img src="/img/icons/main_ico_instant.svg" alt="Icon"><div>{% translate list1 %}</div></div></div>
   <div><div><img src="/img/icons/main_ico_worldwide.svg" alt="Icon"><div>{% translate list2 %}</div></div></div>