Extract wiki and external links, file and template usages from text.

Store said "References" in a table, and append to MW standard links tables.

In order to display a link to the post on the Flow board (e.g.
  (file link; _from a post_)"), this needs the core change I679abfbbdbd
that runs the WhatLinksHereProps hook.

Bug: 57512
Change-Id: If548da37d55f767a16745bc5f72504620883b7d7