Move bulk add to separate controller

We're relying here on the ordering of before filters: `tracks_mappings_progress` is setting `@site`, which is required by `checks_user_can_edit`. This ordering is counterintuitive to me - I think we should be checking whether the user should be able to access this page before doing anything else. But if I move `checks_user_can_edit` to the top, it errors because `@site` isn't set yet.

I think it would be better to have a `find_site` callback here as we do in [`ImportBatchesController`](https://github.com/alphagov/transition/blob/master/app/controllers/import_batches_controller.rb#L4) for consistency and explicitness.