Require that ephemeral posts live for at least one hour

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.

test/activity_expiration_test.exs

@@ -18,4 +18,10 @@ defmodule Pleroma.ActivityExpirationTest do
     assert length(expirations) == 1
     assert hd(expirations) == expiration_due
   end
+
+  test "denies expirations that don't live long enough" do
+    activity = insert(:note_activity)
+    now = NaiveDateTime.utc_now()
+    assert {:error, _} = ActivityExpiration.create(activity, now)
+  end
 end