Branch v2 1242 force shib login

For institutions with force_shib_login flag enabled users that belongs to these institutions are not allowed to login or register locally - forcing them to use CAFe.

refs #1242
Can this be done inside the method `authenticate!` as we have for the [local authentication check](https://github.com/mconf/mconf-web/blob/branch-v2/lib/devise/overrides.rb#L14)?
It would be better to have this checks in a single place.
(That `overrides.rb` file is not in this branch yet, but you can copy it here and adapt.)