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
* Added a fixture `posts:mu` that creates 4 users belonging to 4 roles, 50 posts that have authors evenly distributed, 5 tags and all 50 have one tag attached to it, evenly distributed.

Caveats / questions

* Started testing
Merge pull request #3371 from javorszky/iss3100

Implement user and related content deletion