Move mediawiki.legacy.* modules from skins/common/ to resources/

Also renamed some image files for consistency and sanity.

Added temporary symlinks to the new location for ajax.js and
wikibits.js, which still seem to get many hits for unknown reasons.
I'm going to figure this out and remove them before 1.24 release.

shared.css also appears in the access logs from bug 69277, but with
many fewer hits (it might just correspond to debug=true requests)
and failing to load a style file is a lot less problematic from
user's perspective than failing to load a script.

Bug: 69277
Change-Id: Id4a8f9470d287c73309ff526ffb88984e2d351b9
Merge "Move mediawiki.legacy.* modules from skins/common/ to resources/"