Add Rubocop

Starting to bring over changes from #2501 in more manageable pieces.

Adds [Rubocop](https://github.com/bbatsov/rubocop) to the development requirements, for now the auto-generated config gives Jekyll a pass because it has disabled the offences. These _todo_ offences will be re-enabled in following pull requests once they have been fixed.

The Rubocop task has been added to the default rake task so it will be run after the tests and features in the CI build.