DELETE endpoint to delete event(s)

My suggested API:
- Remove `DELETE /events/` endpoint
- Add `POST /events/delete/by_ids`
- Add `GET /events/delete/by_period?start=X&end=X`

Reasons why the old delete API is bad and should be replaced completely:
- The new "by_ids" endpoint makes it possible to remove multiple specific events in one go while the old API could only delete one event at a time.
- The old DELETE endpoint is also broken because both `/events/` and `/events/count` are in the same namespace

Things which need to be considered:
- Vacuuming the database

Resources:
- Discussion with @leoschwarz in aw-server-python PR https://github.com/ActivityWatch/aw-server/pull/57