Allow override of page disply within CategoryViewer

Allow override of page disply within CategoryViewer

Patch adds two hook which are described in hooks.txt.  This
is being used to allow Flow to offer two links instead of just
one that are relevant to the page that was categorized.

The default output without these hooks is:

   <a href="...">Topic:Soiasdf90f09</a>

This patch allows flow to provide context as to where this topic came
from, by replacing that with:

   <a href="...">Topic:Soiasdf90f09</a> on <a href="...">Talk:Help</a>

(Note that the names of pages within the Topic namespace will also
become more friendly soonish, but outside the scope of this patch).

Bug: T87793
Related-Flow-Change: Ia4f2953bcd807ba3990e762a2efcaab428c40147
Change-Id: I182e6e35fcc3a2a298e928e088579bdb22e145ff