Support colon link escapes for categories and files

This commit adds bidirectional support for prefixed colons in
internal link targets, such as the ones required by links to files
and categories. It automatically adds a colon to links targeting
pages in the File and Category namespaces (as determined by mw.Title)
and allows an optional colon prefix for all internal links after
the optional whitespace.

Bug: T38227
Change-Id: Ie2801c42da4238555ad7c6544a8412610a528597