Consider Making event.expire_at Not Nullable

[allella]

@bogdankharchenko this came from #726 (comment) where we did a decent amount of tweaking to tests and scopeFuture() based on event.active_at and event.expire_at.

I forgot to ask you if we could do a migration to make expire_at not nullable, since it seems we practically expect events to have an end date. If for no other reason, I wonder how events with no end date would break the calendar view.

Here's my comment from the last conversation about it.

I was also wondering if we should do a migration to make events.expire_at not a nullable field since events should probably always have an end date, even if we're forced to add some number of hours on import. I think we're currently adding 2 hours for Meetup or Eventbrite on import because one of them doesn't send the end date in the their API.

If we required expire_at, I suppose we could make the migration update any existing records with null expire_at to be equal to active_at + 2 hours. It looked like only older events had a null value for expire_at.

[allella]

If we end up doing this, let's also remember to remove or tweak any related tests, like test_events_this_month_falls_back_to_active_at_when_expire_at_is_missing() in
tests/Feature/Http/Controllers/HomeControllerTest.php