Fix Wikimedia overrides of core messages

* Set up an overrides system using MessageCache::get hook
* Use this for createacct-helpusername, createacct-imgcaptcha-help,
  and sidebar
** Rename the version of these keys in this extension to:
*** wikimedia-createacct-helpusername
*** wikimedia-createacct-imgcaptcha-help
*** wikimedia-sidebar
* Small documentation updates

Bug: 65514
Change-Id: Ica73423cc8342f6d9290f287ddeb35ff59d04d8b