Show locale-appropriate content on activity feeds

https://www.pivotaltracker.com/story/show/55800968

Updates the activity on policies and worldwide priorities to only list content that has been translated into the current locale. Does the same for the policy/announcement feeds on people pages as well.

Note that I have converted the tests for `RolePresenter` and `PersonPresenter` so that they no longer inherit from `PresenterTest` as it disables database queries - nice for faster tests in theory, but it makes testing our tangled domain model really (really) hard.