Flow/Link table spec

We want things in Flow posts to cause entries to be added to link tables.

Requirements

 * Linking to a page or URL, including an image or a template, should cause entries to appear on Special:WhatLinksHere, Special:LinkSearch, and in usage lists for files.
 * In particular, including a file needs to add to the GlobalUsage tables.
 * It may need to be eventually possible to add a Flow board to a category.
 * It is very preferable that we can figure out exactly which post includes a link/file/template.