quotas for open registration and expiry of pending accounts #82

[limits] caps pack, blob and asset sizes but nothing per account. The Roadmap's decision text lists quotas as a precondition for opening the instance and they did not land. Pending accounts that never verify are never expired.

• limits.max_repos_per_user and limits.max_bytes_per_user, with an admin override per user.
• A reaper that deletes accounts still pending after a configurable number of days.

Ref #14