Create lockdown test for tag cloud

The tag cloud appears to have 100% code coverage, but it turns out that
there are no assertions regarding this part of the code, it just runs
without raising any exceptions.

Run the lockdown test with `bundle exec rake test:lockdown`.

The lockdown test is an `approval` test, meaning that whatever you
previously approved is considered the correct result.

The first time you run the test there is no previously approved result,
so the test will fail.

To make it pass copy the `.lockdown/received.html` file to
`.lockdown/approved.html`. By doing this you _approve_ the current
output as being correct.

If you run `bundle exec rake test:lockdown` again, the test should pass.
Copied get_stats_tag out to a new model.
Extracted all get_stats_tags instance variables to model.
Renamed current user to user.
Rename some variables.
Rename cut_off_3months to cut_off.
Extracted sql for 90days to private method.
Extracted base sql to private method.
Refactored private SQL methods into one.
Made cut_off an optional argument. Removed references to 90.
Updated views to use new instance variables.
Refactored view to not have cut_off hard coded.
Added tags method to supplant @tags reader.
Removed tag_ prefix from tag instance variables.
Break all instance variables into methods.
Changed view to use helper for font size.
tags method now calls private compute tags.
Merge branch 'iteration3'