Issue #1992744 by quicksketch: Add a dedicated @Editor plugin annotation.
Issues #1987602, #1978894, #1978892: Migrate ajax form callbacks to controllers.

  - ajax_form_callback()
  - file_ajax_upload()
  - file_ajax_progress()
Issues #1987602, #1978894, #1978892: Fix dumb namespace error.
Issues #1987602, #1978894, #1978892: Remove file include since it's not
supported for routes and doesn't appear to be needed.