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