Signup plugin + ordergroup approval

This plugins adds a public signup form. Prospective members can enter their details, and a new user + ordergroup will be created. Access for this ordergroup is restricted, until it is approved by an administrator.

This is the first plugin to use [deface](https://github.com/spree/deface) for overriding existing views. Please see the [wiki page on plugins](https://github.com/foodcoops/foodsoft/wiki/Plugins) for a bit more information.

This approval can also be done by other plugins, e.g. after an online payment of the membership fee.