Extract code for UC's Central Login authentication into it's own gem

[hortongn]

(moved from uclibs/scholar_uc_legacy#560)

Descriptive summary

We will likely eventually want to use the code for Shibboleth authentication in other apps. We should see if we can extract the bulk of the logic into a separate gem that will let us use it in other rails apps that use Devise.

Assumptions:

• We may want to use this with non-Samvera apps
• Much of what is in CallbacksController#shibboleth can be moved to a gem
• Much of the changes we made to views in the app can be left in the app
• The gem would have omniauth-shibboleth as a dependency
• The gem would be specific to UC's authentication so it will grab the attributes that UC provides (first name, last name, department, etc.)
• The gem will handle properly logging out of Shibboleth when logging out of the app.
• Everything will be well tested.