use build id instead of app id for the submit url

The server already accepts a build id in this location. It recognizes it as such and uses it to look up the app_id, thus tagging the form submission with both an app id and a build id. This will let us do some nice stuff like detect which exact build a submission came from (and thus what build is on the phone). This will also be used in the upcoming "readable form data page" feature.