Show all three options for filtering system note activity for issue and merge request

• Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/26723 to show all three possible options in a dropdown:
  • Show all activity
  • Show comments only
  • Show history only (i.e. this would be everything that is not a comment)
  • Applies to issues and merge requests.
  • BE choice of the three options needs to be updated because now you have a third choice.

If you are in Show history only mode, then you should not be able to add a comment. (For merge requests, that would be inn the Discussion tab only.) This is further addressed in https://gitlab.com/gitlab-org/gitlab-ce/issues/51323.