"Confirmed" scope for users; cleanup unused variables; clickable gravatars everywhere

- We added a "confirmed" scope to the User class, replacing the ad-hoc confirmed() and find_confirmed() methods
- the recent_updates and new_crops controller variables were unused; we deleted them
- gravatar + username extracted out into a partial
- call this partial from everywhere user icons were needed
- in particular gardens/show ([story 41638881](https://www.pivotaltracker.com/story/show/41638881))
- make the user picture clickable ([story 41366245](https://www.pivotaltracker.com/story/show/41366245))