Follow on from #602

• We calculate approximate flight time.
• If they are using a drone that has it's battery time saved at a DroneTM stat, we should use that!
• Obviously the user doesn't want to use a DJI Mini 4 Pro battery of ~20 minute flyable time, when the task area is 22 minutes.

Note this could also be done automatically? But it might not be intuitive to do this. Perhaps a user wants to lock and test creating a flightplan, but then unlocks. We would have split the area based on their drone model / battery life.