option to enable ldap only login screen

Created by: alerque

Re feature request #605 (closed), moved here.

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.