Implement traffic drop notifications

Changes

This PR implements traffic drop notifications, similar to already implemented spike notifications.

Postgres migration included that extends the spike_notifications table with a type field indicating whether the notification is tracking drops or spikes. Table name should be probably left intact - since we don't have a way to run a migration renaming it.

[ ] Extract migration to a separate PR once review is concluded

It's important we don't send out drop notifications if e.g. traffic is barred via payment issues, hence the worker looks owner's accept_traffic_until up.

BTW the PR makes it no longer possible to set a negative threshold.

Tests

[x] Automated tests have been added
[ ] This PR does not require tests

Changelog

[ ] Entry has been added to changelog
[ ] This PR does not make a user-facing change

Documentation

[ ] Docs have been updated
[ ] This change does not need a documentation update

Dark mode

[ ] The UI has been tested both in dark and light mode
[x] This PR does not change the UI

plausible / analytics