Show users who have created a tag

## Feature Request Form
### Problem you are trying to address with this feature
With the new activity pages, if you are searching just for a tag, i.e. `tag:privacy` you'll see a list of documents that contain the tag, but you won't see a list of the users that have tagged with it in the sidebar. Adding the users panel to this view would be a powerful way to search tags and ask 'who?'.

### Your solution
Introduce the user panel in the sidebar when a user is doing tag or url searches (it's already there for groups, and N/A for a single user profile). It should be frequency sorted and probably only show the top X results.