Handle deletion of song parts

[glynnbird]

As laid out here: https://github.com/orgs/Choirless/projects/3#card-43852025, the front end will soon delete a song part's .webm object from the raw COS bucket when a user deletes a part.

New triggered actions are required on various buckets so that a deletion from the raw bucket triggers the subsequent deletion of all other derivative artefacts, both audio and video.

A further trigger is required so that the whole chain of deletion events causes the final video to be rendered again (without the deleted song part).

[glynnbird]

Sean reckons the front-end is already deleting the .webm file on deletion of a songpart, so we're good to go triggering "on delete" actions.