Feature facebook single sign on+3

This is a cleaned up version of: https://github.com/liqd/adhocracy/pull/778

Matthias concerns have been addressed and a bug which allowed users to connect multiple adhocracy accounts to one velruse account has been fixed.

Updated summary:

Velruse is used to authenticate the users.

Includes:

- configs, so the velruse service can be controlled with supervisor
- creating new users with the Facebook login
- connecting existing users with Facebook, allowing them to use the Facebook login, too
- a forward and backwards migration for the the table which holds velruse users
- unit tests on the model of velruse users
- integration with the openid login interface
- possibility to use velruse for other services (openid, twitter...)

In the course of integrating the Facebook login with the openid login, I intruded an openid html file. Its nothing bad, the inner workings of the openid login have not been touched.
I did this because tweaking the openid JavaScript plugin to work for me seemed as hard as replacing openid with velruse altogether.

You need to add Facebook keys to velruse.cfg.

Facebook single sign on is turned **off** by default.