Increase Event test coverage to 100%

Some incidental refactoring to achieve this goal:
* Event.search_tag_grouped_by_currentness had some duplication with Event.ordered_by_ui_field, so remove the duplication so that we don't have to write the same tests twice.
* Event#time_range was completely unused, so it got killed.
* EventController specs were reaching inside the Event model and mocking its internals, making them unable to change, rather than mocking its interface.