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.
Update selenium webdriver gem
Extract get_stats_tags computation into model layer
Make levels variable a constant
Extract tag fetching into a private method
Extract tag cloud min and max calculation to a method
Get rid of compute method, make tag cloud do one calculation only
Refactor tags partial to not use instance variables
Refactor cloud tags even further to not use as much duplicate code
Simplify tag cloud max calculation
Clean up tags font size rendering