Editing scenarios - Add default values before editing values

This feature is already built in, but it uses the values from the scenario defaults as the placeholder text. If we actually set the value, the value will be saved for any field that isn't modified at the scenario level. This will create a duplicate setting at the scenario level in the backstop.json file. It won't negatively affect operation, but we don't want the scenario_defaults values being duplicated at the scenario level.

This issue/feature is complete in the new version.