Change login/createaccount forms to new appearance

If a global variable is set or if you add ?useAgora=1 to the query
string, Special:Userlogin loads a different login or create account
template (Userlogin-/UsercreateAgora.php) with an Agora look and
different messaging.  Otherwise the current form is unchanged so
that wikis can cut over to the new look when desired.

These new templates apply mw-ui-formlist and mw-ui-button styles defined
in a new 'mediawiki.ui' CSS module in core (copied from Extension:Agora).

In useAgora mode, Special:Userlogin also:
* Adds new modules with some additional CSS for new form features
  ("Join wiki", benefits of creating an account).
* Defines new "userlogin/usercreate-xx" messages, many are the same as
  existing messages but without ':' on the end.
* Uses a distinct title for each mode instead of generic "Log in /
  Create account".
* Uses JavaScript to munge the create account CAPTCHA.
* Outputs checkboxes using UserloginTemplateAgora::labelledCheck()
* Displays a benefits column of wiki edits/users/contributor numbers.

TODO:
- Restyle/reposition language selector
- Munge CAPTCHA in PHP not JavaScript, i18n of new CAPTCHA messages.
- Identify the subset of Agora appropriate for non-Vector skins and
  create mediawiki.ui.default.css from that.

Patch set 18: Agora styles now in core.

Bug: 44628
Change-Id: I859edab4fc4fa9fe35fdef15fc429ae19a95305d
Merge "Change login/createaccount forms to new appearance"