Projects: improve deletion

[stsewd]

For some related fields, Django first does a select, then it probably loads each object into memory? But after that it does the deletion using a IN statement with the ID of the objects...

Using raw_delete avoids that, but it can also cause integrity problems, as Django does its own implementation of on_delete, so I chose models that don't have additional relations and also shouldn't have problems as they are always deleted when the project is deleted. Another source of problems is when models have foreign fields with on_delete=SET_NULL, as django will do an UPDATE over the model with all the related fields... for example for latest_build, django will run an update over all projects with an IN (... all builds IDs..), to set the latest_build ID to null..., which is silly since the whole project model is being deleted, but django doesn't know that lates_build can only contain a build from the project. A raw delete could help, but builds and versions have a lot of related fields in other models, so I'm a little hesitant.

Closes #12410
Ref #10040

[ericholscher]

This looks like a good test to see if it helps, then we can expand the logic. Appreciate the PR description, very good summary of what I was trying to figure out the best path forward on. 👍

[ericholscher]

Good note 👍