Improve the handling of colon-prefixed wikilink targets

* Removes the colon from the link text of simple wikilinks (fixes bug 43171)

* Properly respects the colon for links to category pages

* Fixes round-tripping for links like this:

    [[:Category:foo.png]]
    [[:Foo:{{echo|bar}}|e''foo'']]
    [[:Foo:{{echo|bar}}]]

Overall one more wt2wt, one more wt2html, and four more selser parser tests
are passing.

TODO: add a test for the link to a category page, as this did not trigger a
test change.

Change-Id: I83e17acab44ccbd19deb4352ad51536a40a11aaa
Merge "Improve the handling of colon-prefixed wikilink targets"