Removes head_tag_content override to pick up Google Scholar tags from Sufia. Fixes #620.

This work required first getting ScholarSphere working on top of Sufia 4.1.0, which included proxy- and transfer-related code ported from ScholarSphere, so this meant a lot of deletion and cleanup. Then I needed to tweak Sufia to allow downstream head content injection, which is sitting in a branch awaiting a PR merge here: https://github.com/projecthydra/sufia/pull/714 The last step was creating a new override in ScholarSphere to inject the favicon_link_tag that isn't in Sufia. A new spec test is included to make sure the favicon exists.

:bangbang: If you want to wait for the Sufia PR to be closed first so our Gemfile points at the master branch rather than a short-lived feature branch, that's cool -- I can tweak the Gemfile and Gemfile.lock once that happens, squash, and force-push the changes here.
Done.