Add feature to (optionally?) hide login methods

Separating this feature from #795574: Port Maintenance Helper to D7

Also added features for hiding all user paths apart from user/password (so the only way to log in is via a one time login link) and removing the user login block (which only works with mongodb_block.module at the moment, need to add support for core blocks too.

Attached is the patch in that issue -- it currently contains some porting-to-D7 code that may need re-rolled if that issue lands first.