Refactor login hook into MobileFrontend.hooks.php

* Also remove now unnecessary ExtMobileFrontend::attachHooks()
* Changes 'OverloadTemplate' class to account creat/login-specific
abstract class, which felt more appropriate since we had nothing
else using OverloadTemplate, and we need a place to put shared logic/
handling for login/account creation-specific stuff, and it seemed
silly to create yet another class. The naming of these classes
could probably use some improvement, however...
* Exposes RequestContext in abstract template for login/account
creation templates
* Moves data prep for anon watchlist addition after login to template logic

Change-Id: I70bd611b6386e0e1b7f0414fec3decef0e716ac7
Merge "Refactor login hook into MobileFrontend.hooks.php"