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.
added stats/TagCloud model
moved code from stats_controller#get_stats_tag to TagCloud object
clean up variable names
extract sql query strings to private methods
combine sql builder methods
remove all duplication in TagCloud and instead use 2 tag clouds
view accesses data through tag cloud objects
remove duplication in the view by passing in tag cloud to a local variable
move tags compuation to custom accessor
clean up TagCloud moving all computation into accessor methods and removing compute
clean up the stats view by pushing calculations into TagCloud and a helper method
Refactoring of stats tag cloud