Make MediaWiki:Redirectpagesub accept all wikitext.

Make MediaWiki:Redirectpagesub accept all wikitext.

Redirectpagesub is used as the system message for subheadings on redirect pages.
Currently, it only performs transformations of {{-style constructs whereas it
could be useful to accept a wider range of wikitext. For example, one could change
the message to "Redirect page ([[Special:WhatLinksHere/{{FULLPAGENAME}}|links]])"
so that the subtitle displays a link to Special:WhatLinksHere. Currently that does not work.

The subtitle is already in a block-level <div> so it can already handle additional
block-level elements, thus the inclusion of arbitrary wikitext shouldn't break things (too) badly.

Change-Id: Icbd29d71c325c63dfda80dfcd52ed32358a2f24e