Use non-deprecated moment.substract()

Closes #4545
- Go with subtract(value, string) instead of subtract(string, value)
Merge pull request #4548 from felixrieseberg/iss4545

Use non-deprecated moment.substract()
Do not display "auth failed" modal on clean posts.

Refs #4543
- Check EditorController.isDirty before displaying the
  authorization failed modal dialog.  This prevents the modal
  appearing immediately upon entering the editor in cases where
  auth has failed prior to opening the editor.