mediawiki.jqueryMsg: Implement HTML support and improve HTML escaping.

mediawiki.jqueryMsg: Implement HTML support and improve HTML escaping.

* Whitelisted elements.
* Whitelisted attributes allowed on all elements, and additional ones
  per element.
* Currently, only b (bold) and i (italics) and the common attributes are
  implemented, but more can be added.
* Treat error message as text
* Implement appendWithoutParsing as an extra safeguard to ensure
  jQuery never implicitly parses HTML.  Reuse this where a similar
  case was handled one-off.
* Rename some of the link functions and variables to be clearer about
  relationships.

Bug: 44525
Change-Id: Id8902af9568092d0318d0ac9d1ca6c9c195d36fe