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 TagCloud model
Move get_stats_tags work to TagCloud
Improve TagCloud method names
Extract SQL queries in TagCloud
Remove duplication from TagCloud
Use TagCloud objects directly in views
Remove view duplication
Extract tags method in TagCloud
Extract methods in TagCloud
Extract relative font size calculation
Default to capybara-webkit instead of selenium