mediawiki.jqueryMsg: Refactor handling of replacements/parameters in links

mediawiki.jqueryMsg: Refactor handling of replacements/parameters in links

You should now be able to use replacements/parameters (such as '$1')
anywhere inside both link text and link target, in both external links
('[http://foo/ Bar]') and wikilinks ('[[Foo|Bar]]').

As a side effect of various cleanups, HTML in link text is now preserved
in all cases and is never wrapped in <span class="mediaWiki_htmlEmitter">.

Added test cases for it all.

Bug: T49395
Bug: T50064
Change-Id: I56d8f7ec03a70f5c2360d9c5099496ecb2f668ad