Separating titles for same show on consecutive days

When a show (usually The Frequency) airs twice in a row without any shows in between, the titles submitted for the second day are co-mingled with those from the first.

I just modified the `group_by_show` class method of `Suggestion` to also split up titles (i.e. create a new `SuggestionSet`) if there is more than 18 hours between title suggestions for the same show.