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.
Extract TagCloud model
Move instance variables to TagCloud model
Rename TagCloud instance variables
Extract sql_90days method
Extract sql method
Remove duplication in sql methods
Remove duplication in tag clouds
Replace instance variables in views with TagCloud instance variables
Use one `_tags` partial per TagCloud
Extract `tags` and `levels` methods in TagCloud
Extract `divisor`, `tag_counts`, `max` and `min` methods in TagCloud
Move tag size calculation out of _tags partial