Bugfix: Add AnonymousFeedbackAdmin

Hey,

In the last pull request I didn't fix the problem of showing anonymous feedback correctly. I added anonymous feedback links, but they don't show the correct feedback (they show authenticated feedback instead of anonymous feedback). This occurred because I was registering the FeedbackAdmin, which didn't implement any functionality to show anonymous feedback.

I added a new AnonymousFeedbackAdmin and now it (really!) works.