Show locale-appropriate content on activity feeds

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.