Support LDAP login tabs wrapping to second line (and a few other login fixes)

Fixes some regressions introduced for customers with multiple servers configured for login and/or long label names.

Also, improves styling for the login page on small screens.

See the bad behavior here: https://gitlab.com/gitlab-org/gitlab-ce/issues/23435#note_17117893

https://gitlab.com/gitlab-org/gitlab-ce/issues/23435#note_17117677

See merge request !6993
Signed-off-by: Rémy Coutable <remy@rymai.me>