Prevent conflict when using more than one SiteTreeURLSegmentField in a CMS edit form

When using multiple SiteTreeURLSegmentField's in the same edit form the Title field was updating them all (typical case when translating URL's). This fix ties the "update from title" behavior to the primary SiteTreeURLSegmentField (name=URLSegment) and only adds the "update" button to the Title field if required. Also changed `.parents('form')` to `.closest('form')` to ensure we only have one element returned in the jQuery collection.