Less visible options when connecting a friendica contact

* When you add a friendica contact from another friendica server and your address is provided via an URL parameter, then most options on the "dfrn_request" page are now hidden, to make usage less confusing.
* If a user is already connected to another user, it doesn't make sense to display a "connect" button on his profile page.
* Notifications via mail now have some additional headers for better filtering. Additionally the threading now works, since the "parent" parameter is provided when calling the notification.
* Vier: The "plus" style is changed a little bit to improve it.