Allow login with passwords not meeting complexity requirements

Allow login with passwords not meeting complexity requirements

As part of https://www.mediawiki.org/wiki/Requests_for_comment/Passwords

This patch:
* Allows users to login with a password that does not meet the
  requirements of User::isValidPassword(), including the minimum password
  length.
* Adds a configuration flag that specifies if users should be sent to
  the change password form when they login with a password that doesn't
  meet the requirements of User::isValidPassword().

To test the UX as it will be on WMF wikis, set
$wgMinimalPasswordLength=6 in your LocalSettings.php.

Change-Id: Ib7b72005fea1c69073c0a33a68c0a0df0d6528d2