Automatic login with ownCloud user credentials

This adds an admin option to enable automatic login with RainLoop (as per idea in #248)

 If this is enabled, then:
- the user ability to configure the login details is removed
- the ownCloud login username is used as the RainLoop email address
- the ownCloud login password is used as the RainLoop password

Tested with ownCloud 7.0.1 on Debian. In my setup I am also using the user_pwauth ownCloud plugin to link ownCloud to the user accounts on the system.