WIP: Add RuboCop style and lint checks

Greg mentioned that it could be nice to use [RuboCop](https://github.com/bbatsov/rubocop) for the Rails app. This adds the very basics required to run the `rubocop` tool. I haven't fixed any of the reported "offences" yet.

The `rubocop-todo.yml` configuration file was generated by `rubocop --auto-gen-config`. The point is for the user to remove these configuration records one by one as the offences are removed from the code base.