Propagate the app id to the gears

@danmcp - as per discussion on Friday - this explores a new optional param on the gears APP_ID.  Needs a migrator still.  REST API continue to returns legacy UUID as 'uuid' and mongo _id as 'id', which is a slight change from what was published last sprint.  Once APP_ID is present, then gear code that needs to rest call back would use APP_ID