Allow moving OmniAuth above password login

Problem to solve

We are trying to encourage users to log in with OmniAuth instead of LDAP (we will eventually take LDAP login away). We want to move the OmniAuth login options above the password login options so users will be more likely to use it. This currently requires changing https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/devise/sessions/new.html.haml and can't be changed in configuration.

Intended users

All users

Further details

Not a HAML expert, but view will likely need to be split into multiple parts and the order will be configured from gitlab.rb or in the admin panel.