Add functionality for translating other document types

This adds the admin functionality to translate News articles, Speeches and
Case studies.

While the implementation was relatively trivial, I think the most interesting
change is in the introduction of the Admin::GenericEditionController. We
should be able to use this test-only controller to reduce the total number of
functional tests we have, while retaining the same confidence in our code. See
the "Introduce GenericEditionsController" commit for further info.

[Pivotal Tracker story 44794809][1]

[1]: https://www.pivotaltracker.com/story/show/44794809