Also pick up links to non-existant files (for LinksTable)

On my local machine, LinksTable tests had been failing for like ...
forever.

The reason for that is that I don't have Foo.jpg & Image.png on my
local setup, which the tests use.
Parsoid treats non-existant images differently, which is why the
reference extraction didn't pick them up.

Looking at default MW behavior, it seems that links to non-existing
files are also shown in Special:WhatLinksHere, so we should as well.

Change-Id: I04d1849a889a4ff5cb1ede76df6c01df8088dc13
Merge "Also pick up links to non-existant files (for LinksTable)"