option to enable ldap only login screen

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 explanatory name and moves it to
the requested location in the configs. The original commit was rebased on master and another was added to give the setting a decent name.