commands.import.airspace: adopt xcsoar's airspace class parsing

This will make it easier to use xcsoar's airspace intrusion code.
model.airspace: use xcsoar for airspace infringement detection

In general this is twice as fast as the postgis approach, speeding
up to 5 times for flights through many airspaces.

As a side effect, xcsoar already knows how to handle flight levels
or altitudes above ground.
model.airspace: return uppercase airspace references

And fix the SI units in get_airspace_infringements
model.airspace: replace NOTAM and UNKNOWN airspace references

XCSoar doesn't know them. Replace with MSL for now...
mapserver: update airspaces
airspace.map: update airspace class names
model.airspace: check configured airspaces only
model.airspace: use QNH if available
Upgrade celery to 3.1
model.flight: commit changes to db after update_flight_path()
commands.flights.analysis: implement generic flight selector helper
commands.flights.copy_flights: use selector helper
commands.flights.delete_flights: use selector helper
commands.flights.find_meetings: use selector helper
commands.flights.update_flight_paths: use selector helper