Plugin: signup + ordergroup approval (for 4.1)

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.

The approval is done automatically when the membership fee is payed. When using an online payment plugin, this is done without a manual action.

- [x] document deface usage
- [x] merge master
- [x] make tests work with plugin enabled
- [x] bring in any updates from foodcoop-adam-beta branch
- [x] rebase, cleanup and bring up-to-date with master
- [ ] bring in any updates from foodcoop-adam master branch
- [ ] test (notably, Rails 4)