mw.loader.store: temporarily allow users to opt-in via cookie

To facilitate mw.loader.store testing on production wikis, it would be good to
allow developers and JavaScript-savvy users to opt-in for mw.loader.store
testing by manually setting a cookie in their browser. With this patch in
place, this would be possible to do by executing the following statement in a
debug console:

    $.cookie( 'ResourceLoaderStorageEnabled', 1, { expires: 7, path: '/' } );

This will be removed from mediawiki.js (long) before the 1.23 release.

Bug: 56397
Change-Id: I51cfd563734a3aeed3667fbb19bf3be40fabbb6b
Merge "mw.loader.store: temporarily allow users to opt-in via cookie"