User: Mostly remove password handling

User: Mostly remove password handling

AuthManager is coming, which will make it easier to add alternative
methods of authentication. But in order to do that, we need to finally
get around to ripping the password-related bits out of the User class.

The password expiration handling isn't used anywhere in core or
extensions in Gerrit beyond testing for expired passwords on login and
resetting the expiry date on password change. Those bits have been
inlined and the functions removed; AuthManager will allow each
"authentication provider" to handle its own password expiration.

The methods for fetching passwords, including the fact that mPassword
and other fields are public, has also been removed. This is already
broken in combination with basically any extension that messes with
authentication, and the major use outside of that was in creating
system users like MassMessage's "MediaWiki message delivery" user.

Password setting methods are silently deprecated, since most of the
replacements won't be available until AuthManager. But uses in unit
testing can be replaced with TestUser::setPasswordForUser() immediately.

User::randomPassword() and User::getPasswordFactory() don't really
belong in User either. For the former a new PasswordFactory method has
been created, while the latter should just be replaced by the two lines
to create a PasswordFactory via its constructor.

Bug: T47716
Change-Id: I2c736ad72d946fa9b859e6cd335fa58aececc0d5