The defaults that were in $wgMediaHandlers are now listed in
MediaHandlerFactory.
The main advantage of doing this is we get O(1) replacement when
extensions set a media handler in their extension.json.
Bug: T141305
Change-Id: I05771a673837ab8d6331eedc24eb707be7f3a250
This will allow further refactoring of override logic in parser tests.
Ideally the factory class would not use $wgMediaHandlers directly, but
that ends up breaking too many tests for now.
Change-Id: I34a63ee7089ff26f86f3dd6f3cd1a37928bc4005
