Move event sorting label

`Event.sorting_label_for` was only being used in `EventsHelper`. For general cohesion it didn't belong in `Event` but where it is being used in `EventsHelper`.

In doing so, I noticed that tests that should have failed when calling `Event::sorting_labels_for` continued to pass. By including modules into the global name space i.e. `include EventsHelper` it would allow one to call `Event::sorting_label_for` or `AnyValidModule::sorting_label_for` which seemed a bit unclear. Updated the `Helpers` to not include modules for this reason and make calling helper methods more transparent.