Displaying the author of a post on the "display" page/network page as default page after login

This pull request contains solutions for the following issues:
* https://github.com/friendica/friendica/issues/982
* https://github.com/friendica/friendica/issues/1073
* https://github.com/friendica/friendica/issues/1059

The major change of this pull request is a changed behaviour of the "display" page. By now it showed the profile of the current user. (or the owner of a post when not logged in)

Now it shows the author of a post and a link to his profile page. In the future it can be extended to more profile data like the location or a general description. It also contains a link to a possibility to add this contact (if it was a friendica or diaspora contact) if the current user hasn't already added this contact.