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.
As a separate chunk of work, it might be worth looking at whether MOBILE_COOKIE_NAME is actually used anywhere outside of the method in ApplicationHelper and in this spec. If it's not, calculating it inline in both places seems more reasonable. That doesn't really affect this PR though :)