When Enter is pressed in a form, the browser will find the first
submittable element and trigger it. By default <button> elements
have type set to 'submit' which causes them to be targetted by
this behaviour.
Adding `type="button"` prevents this behaviour. This change
targets some main offenders - in particular the markdown toolbar.
There are of lots of other `<button>` elements without this attribute
set, so they need tidying up. Not currently aware of any others that
exist in a <Form> so may be immune from this behaviour.
Any editor for the markdown mode will now automatically get
the markdown toolbar added.
The comment node has been updated to handle this properly and
to not add two copies of its content to the sidebar.
