mediawiki.icon: Move arrow icons to a separate module

The arrow icons for the enhanced watchlist/recent changes could be
useful elsewhere. But to do that, we need to be able to load them
separately; this does that by adding a new module just for the CSS
needed for the arrows.

Also, this fixes a use of OutputPage::addModuleStyles that really should
have been OutputPage::addModules so dependencies are resolved.

Change-Id: I639a5ccbc3b7b2794e7b12ecb64e5ee6b10a640a
Merge "mediawiki.icon: Move arrow icons to a separate module"