Fix non-responsive buttons for managing feed readers
In recent Firefox releases the "explicitOriginalTarget" property returns the
associated form element instead of the originally clicked button so that the
buttons for managing feed readers on the option page no longer worked.
Determine the button used to submit the form using the "submitter" property of
the new SubmitEvent instead.
Add a polyfill for older Firefox releases not yet supporting this.
In recent Firefox releases the "explicitOriginalTarget" property returns the
associated form element instead of the originally clicked button so that the
buttons for managing feed readers on the option page no longer worked.
Determine the button used to submit the form using the "submitter" property of
the new SubmitEvent instead.
Add a polyfill for older Firefox releases not yet supporting this.
