add option to disable standard login if other methods present

New config option allows the standard login system to be disabled when
other login methods are enabled (i.e. only enabled methods will show on
the signin page).

See feature request #605, moved to feedback board here:

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

Originally 1399a0d4c64ececf6d77a380d2e5a20529971409, modified with a
more friendly name and rebased on latest release.