Revised design of Special:Userlogin

If a global variable is set or if you add ?useNew=1 to the query string,
Special:Userlogin loads a different login template (UserloginVForm.php)
with the new Vertical Form appearance and different messaging.
Otherwise the current unchanged template renders so that wikis can cut
over to the new look when ready (with messages and links). Once they do
so, the variable and flag will be retired.

The new template applies mw-ui-vform and mw-ui-button styles defined in
a new 'mediawiki.ui' CSS module in core to create a compact vertical
form.  The mw-ui styles specify a Helvetica font stack (that we tested
in the Account creation experiment) in the form area, but NOT if the
user is using some other skin than Vector.

The CSS code is developed using Sass. The patch includes the
Sass scss files, along with a Makefile that uses their Compass build
configuration (config.rb).

The redesigned Special:Userlogin also:

* Displays a "secure login" link if HTTPS is available.
* Loads additional CSS for its form features (e.g. more attractive
  errorbox, "Join wiki" messaging).
* Defines new "userlogin-xx" messages; many are the same as
  existing messages but without ':' on the end.
* Uses a distinct title for Login instead of generic "Log in /
  Create account".
* Removes the [mailmypassword] code branch from its login template as it
  is never executed.

Bug: 44628
Change-Id: I489042c50aa060c90ca18b05097dbe25c4ae6395
Merge "Revised design of Special:Userlogin"