Fix active classes on Markdown preview tabs
    
    When in editing mode, clicking the "edit" tab above the text box causes
    the editor to toggle the classes without checking whether or not the
    editor is already in edit mode. This fixes that by using add/removeClass
    instead of toggleClass.
    
    Fixes: #3991
    
    Signed-off-by: Alexander Scheel <ascheel@redhat.com>