Scopes for Tag Frequency

Ben, I created this branch for the tag scopes we discussed. Now, if we
need to break this down further for things like used_at_least_once for
questions, users, etc. I can create more scopes to that effect as well.
Tag management tweaks

- don't display unused tags in the tag autocomplete field
- master list of all tags and frequency
- link to master list added to bottom of "Everything tagged with" pages