- Adds preview payload button below the Seat Link toggle on the
  Metrics and Profiling admin page.
- Creates SeatLinkData module as a place to store common logic needed
  to construct the payload for Seat Link requests.  Used by
  SyncSeatLinkWorker and Preview payload.
- Moves spec helper method create_current_license to
  EE::LicenseHelpers for reuse.
- Genericize the UsagePingPayload JS class as PayloadPreviewer
  for reuse.
- Add info about preview payload to Seat Link documentation.