Allow users to signup themselves, with admin approval.

* Allow users to create a user account and ordergroup from a publically accessible page `/login/signup`.
* Allow admins to approve new ordergroups.
* Allow admins to create user and ordergroup in one step.
* Optionally restrict access to group_order creation, users and messages to approved ordergroups.
* Optionally show a configurable message on the dashboard when an ordergroup is pending approval.
* Auto-login after creating an account.

All of this is configurable using `config/app_config.yml`, and disabled by default (so that upgrades just work).