Open invitation setting

Implementation of #64 issue:

![2014-02-13 20-33-49 login drone io](https://f.cloud.github.com/assets/365089/2160682/e72b8906-94bb-11e3-8227-ddbe26044ab6.png)

Todo:

 * [x] Fix html in templates
 * [ ] Add migration script for `open_invitation` setting in database