implement facility for admin to approve user generated content

[kra]

We shouldn't have any user generated content which hasn't been approved by an admin to remove offensive or inferior content. We might want arbitrary flags for some things but at first we only need "approved" or null, and that's probably all we will need.

In general we need:

• a way to assign an identifier to the content, this is specific to whatever is being approved
• a way to store and access identifiers and flags. The challenges use a CSV text file which is included in backups and syncing, and which is hopefully using a write mutex of some kind. We only need "add flag to ID" and "list flags for ID".
• a way for all admins to be notified when there is content to review, or when there is a lot of content to review
• a way for an admin to add a flag to content or delete content. If the interface is the phone, this could just play each content in order and prompt the admin for keys to approve, delete, replay, skip

[kra]

This is done for the challenges and could be generalized. The challenge version uses the challenge csv file, which has some drawbacks, another facility could use the csv tooling or do something less klugy like redis or sqlite.