Improve page display title handling for category pages

Improve page display title handling for category pages

One use case of display title is to localise page names with Translate
extension or without. While the page title changes, the subheadings still
say something like "Pages in category Foo/de".

Also converted one raw HTML message to be a parsed message.

First version of this patch caused an issue when previewing
because page title in h1 is not the same as page display title.

This issue is fixed by promoting page display title as it's own member
in OutputPage. Also added getUnprefixedDisplayTitle that attempts to
strip away the namespace prefix to mimic Title::getText() but which
works with display title instead.

Bug: T43720
Bug: T46197
Change-Id: I6097a873297eb57759252fc56ad6d02c44e4c366