Block Users/Items/Attachments

[NfNitLoop]

To control spam, objectional content, and abuse, and to comply with legal requirements, server admins should be able to:

• [ ] Block a user ID
• [ ] Block an Item ID
• [ ] Block an attachment (via its content hash)

Blocking one of those should:

• [ ] Prevent it from being served
• [ ] Disallow POSTing it again.
• [ ] Optionally, delete it from the database.

Additionally, we should have commands to find out the reason that blocked content showed up on our instance in the first place:

• [ ] Command to list which users have attached a particular file. (To detect all users responsible for a particular offense.)
• [ ] Command to list users who have followed a particular user ID. (To detect users who use follows to allow abuse.)