Implement size quotas

[NfNitLoop]

• [ ] Servers should be able to set data quotas for particular users.
• [ ] Servers should be able to set default/maximum data quotas for followed users.
• [ ] Users should be able to set quotas for blogs they follow. (Ex: I want to follow X, but only cache the latest 10MiB on my behalf.)
• [ ] There should be a command to show quota usage.
• [ ] There should be a command to clean up over-quota data. (Right now, we don't preemptively delete it.)

Advanced, possibly future features:

• [ ] Separate quotas for Items and file attachments?