Follow-up from "Instrument viewing merge request diffs file by file"

The following discussion from !48470 (merged) should be addressed:

• @ashmckenzie started a discussion:

thought: We have the following defined at https://gitlab.com/gitlab-org/gitlab/blob/ashmckenzie/fix-editing-group-without-existing-saved-settings/lib/gitlab/experimentation/controller_concern.rb#L85-87 which is handy:

def dnt_enabled?
  Gitlab::Utils.to_boolean(request.headers['DNT'])
end

We should probably extract that out into a separate concern/helper as it's not strictly experiment only and then we can re-use it here 🙂 One for a follow-up 🙂

cc @patrickbajao