MAVLink app: Default to MISSION, not MISSION_INT

In order to ensure correct transmission the mission system needs to default to the legacy protocol and switch to the new implementation when these conditions are met:
  * If the GCS sends a MISSION_REQUEST_INT - it will do this based on the AUTOPILOT_VERSION flag indicating int mission support
  * If the autopilot sends a MISSION_REQUEST and has the AUTOPILOT_VERSION flag for 2.0 set, the GCS should NACK it, which will make the autopilot retry a MISSION_REQUEST_INT
  * If the autopilot sends a MISSION_REQUEST_INT and the GCS does not support it, the GCS will ignore and time out. The autopilot could retry now opportunistically with the old protocol, but this is not great for lossy links.
  * If the GCS sends a MISSION_ITEM_INT - this is a fallback