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.
Create a tag cloud class

This duplicates the code in the controller.

We are calling the new class from the controller, but we are not yet
using the results. This ensures that the code can run without raising
any exceptions.
clean up get_stats_tags method - iteration 3 finished
renamed variables - finish iteration 4
consolidate sql queries - finished iteration 5
refactor tag_cloud.rb to remove 90days mentions - finished iteration 6
changes instance variables defined in controller for view - finished iteration 7
remove instance vars from partial - finished iteration 8
extract method tags
remove compute method - finished iteration 9
clean up view logic - finished iteration 10
Merge pull request #1 from germsvel/iteration10

Merge Iteration10