Redo WhatLinksHere query and add a *_from_namespace field to link tables

* Also tweaked the query so MySQL avoids doing a page_name
  index scan when it should start with the link table index
* Added population script (triggered by update.php)
* Also removed uniqueness from some indexes where it is redundant
* Renamed two confusing variables

Bug: 60618
Change-Id: Icca99b6ae0ef76cb77695faf82c615516191da36