Need_approval: request approval to admin on registration


related Issue: #146
development branch: Nanamachi/mastodon/prototype/private-instance

As in title, I’m trying to add new feature that requests approval to admin on registration.
This may be, as haniawni mentioned in above issue, beneficial for semi-closed community like club or small society.
It almost completed, but I want to approve via URI like confirmation.
I think it can be implemented with extending devise or warden, but how?