give easy to understand name to new config option

Re feature request #605, moved here:

http://feedback.gitlab.com/forums/176466-general/suggestions/3788023-only-allow-login-via-ldap

Pull request #5492 started to implement support for disabling the
standard login screen when other auth mechanisms were enabled. It used a
fairly confusing name for the new config option and a request to rename
it went unanswered. Eventually the PR was closed.

This should give it a much more self explanitory name and moves it to
the requested location in the configs.