Stick with `rubocop:disable` or `rubocop: disable` (a space after colon)

We should make up our mind between rubocop:disable and rubocop: disable. I am not sure if we could write a cop for this though....

The following discussion from !8470 (merged) should be addressed:

• @godfat started a discussion:

  I just realized we sometimes use rubocop: disable and sometimes rubocop:disable 😬 Let's keep this because for CodeReuse/ActiveRecord we're mostly using this, but I never know this will also work.