Issue title: Make an ? Avatar for anon or hidden people
 Issue body: Eg in update feeds

 Comments: 
 Comment 0: Gravatar can already handle this with a 'mystery man' default image.

http://en.gravatar.com/site/implement/images/

 Comment 1: this is now fixed. 

/avatar 

redirects you to the mm default image

{'source': 'GithubIssues', 'repo_name_with_owner': 'theCrag/website', 'issue_id': 1044873, 'repo_stars': 89.0}