This commit updates the plugin to the latest guidelines, as shown in
discourse-plugin-skeleton, which involves moving a lot of the code to
dedicated files, use proper namespaces, use the autoloader as much as
possible, etc.
Reintroduces the migration removed in `cdfc5d4`, and fixes it to work with `query_groups`. Since the code is fairly complex, I moved into a rake task so it can be tested and make sure it works.