Allows logging in as a local user, instead of only CAS, in /admin/

I need to be able to log in if I'm working on DMT in a non-CAS
registered domain, like on my laptop. If there's a better way of doing
this, let me know, but right now this seems like the best way to me.
Merge pull request #187 from ccnmtl/add-local-login-form-to-admin-login

Allow logging in as a local user, instead of only CAS, in /admin/