Hack up UserRightsProxy to support setting preferences of remote users, needed for...

Hack up UserRightsProxy to support setting preferences of remote users, needed for a feature in PrefSwitch I'm about to commit. The way this hacks around UserRightsProxy's 'helpful' behavior of refusing to work with wikis not in $wgLoclDatabases is kinda ugly (I'm open to alternative suggestions), but then this entire class is an ugly hack and should be replaced by something more sane (even brion, its author, agrees with this :P)