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.
Add TagCloud class to handle logic from StatsController#get_stats_tags
Remove clutter from StatsController#get_stats_tags
Remove overspecified instance variables name
Refactor query variables inside TagCloud#compute and create their own helper methods
Refactor: Remove duplication in query string and extracted the query to TagCloud#sql
Refactor: Remove 90days variables references from TagCloud
Refactor: Remove clutter from StatsController#get_stats_tags
Refactor _tags partial and remove unnecessary code
Refactor: Extract method from TagCloud#compute for tags calculations and put it on the getter
Refactor: Extract all clutter from TagCloud#compute and put it on several methods
Refactor tags partial and remove some of the logic from it