User quotas

[pfrazee]

Need to:

1. Assign usage limits on diskspace to each user.
2. Track diskspace used by each user.
3. Provide an API for checking the current stats.
4. Provide an API for querying which users are up against their limit.
5. Automatically send email alerts to users who are nearing their limit.

I suggest we enforce the quotas using the admin dashboard, rather than automatically disabling accounts (#21)