Implement user and related content deletion

Closes #3100

* Introduces `destroyByAuhor`, given a context and an id, it will check if context has permission to delete the user by the id, and then deletes all the content where `author_id` is id, and then deletes the user
* Does multiple checks to make sure user exists

Caveats / questions

* I've added a `.get` route as well for testing. I'll remove that if I get thumbs up from yall
* Not sure what the return value should be on successfull deletion. Currently it's `{user: []}`
* Not sure what to do with a process that might fail halfway through. Deleted all the content, but user remained because of an error.