Use display name in category page subheadings if provided

Use display name in category page subheadings if provided

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".

A display title is now used if provided. There is one questionable thing
what to do with namespaces and display title. In this case I think it is
better to have the namespace prefix displayed (or not) according to what
the site admin wants, because we cannot safely strip the namespace prefix
from display title.

By using the page title from OutputPage, we already get a name which is
safe for HTML with no further processing. The name is passed as a raw
parameter which breaks magic words (if any) trying to access the value.
It is easy to fix this by using FULLPAGENAME magic word instead.

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

Bug: T43720
Change-Id: Ide7d4a9ee5c76b6360b53aefd76a2e17a139173f