Implement MSC3866 (`M_USER_AWAITING_APPROVAL` error code)

[babolivier]

Implement MSC3866 so that a homeserver can be configured to make it so new accounts must be approved before being usable.

An MVP of this implementation would consist in:

• a configuration flag to enable the feature (on top of the experimental flag)
• internal changes to keep track of an account's status wrt admin approval
• an admin API to allow admins to allow/deny accounts
• changes to the existing user APIs to allow checking if an account is approved, and listing all accounts pending approval