DO NOT MERGE - Fix upgrade

The upgrade mechanism, sadly, is using the `drop_ddl.sql` before calling the `ddl.sql` which should recreate the newer versions of the function, but `ddl.sql` didn't include the `queue_classic_notify` function and trigger.

Includes:
- [x] fixing the issue
- [x] add a Rails migration
- [ ] ~~add a CHANGELOG entry~~
- [ ] add a `raise` in the code if something from the data structure is missing (thoughts?)

Closes https://github.com/QueueClassic/queue_classic/issues/233.

@abevoelker: this works for me, can you test it too please?