Update SlideImage getCMSFields to allow for DataExtensions to utilize updateCMSFields

Add $this->extend('updateCMSFields', $fields); to getCMSFields
Update SlideImage to allow for show/hide
Update FlexSlider to remove SliderWidth and SliderHeight options from cms

These options are rarely, if ever, utilized and tend to be more confusing for cms users.
Update FlexSlider reverting it back to the state with the Width and Height options
Update FlexSlider to hide slide width/slide height if the user is not in Admin group
Merge branch 'refs/heads/dev'