If we didn't put some kind of lifetime requirement on these, I guess you
could annoy people by sending large numbers of ephemeral posts that
provoke notifications but then disappear before anyone can read them.
The "expires_at" parameter accepts an ISO8601-formatted date which
defines when the activity will expire.
At this point the API will not give you any feedback about if your post
will expire or not.
Add a table to store activity expirations. An activity can have zero or
one expirations. The expiration has a scheduled_at field which stores
the time at which the activity should expire and be deleted.
