Allow login with pre-rename username and password

Add a feature flag and logic to handle automatic authentication as
"USER~WIKI" for users who have been forcefully renamed during SUL
migration. A user authenticated via this path will end up logged in
under their renamed account and have a flag in their session that
identifies them as having passed through the automated username
mangling path. This durable identifier can be used later to alter the
user's on-wiki experience by providing them with warnings and guidance
regarding the changes to their account.

Happy path test:
* Install MediaWiki-Vagrant
* Enable 'centralauth' role
* Create a new user account (eg 'SulTest')
* Create a second user named '<USER>~wiki' (eg 'SulTest~wiki') with
  a different password to simulate a user who was force migrated
* Set '$wgCentralAuthCheckSULMigration = true;' in LocalSettings
* Login using the username from the first account and the password form
  the second account
* Expect to see the second username as the authenticated user
* Expect to see debug log output on the 'SUL' channel describing the
  automated username change and authentication result

Bug: 67995
Change-Id: I86168c64da2253c96edfb1856e91410bf6bcf7a5