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.
extracting method from StatsController to TagCloud
use the TagCloud in StatsController#get_stats_tags
reduced redundancy in variable names
extract sql query to private method
extract another query to priv meth
dedup sql queries
a seperate 90day TagCloud instance
use cloud in the view, rather than seperate variables
use tags partial once for each cloud
extract TagCloud#tags
decompose TagCloud#compute
extracted TagCloud font size calcs