model/flight: Set locations/timestamps to nullable=False
model.flight: add explicit srid to wkt conversion

Fixes a regression(?) introduced by geoalchemy2
model.airport: add SRID to WKT element
lib.waypoints.welt2000: allow usage of local WELT2000 file
import_welt2000: allow usage of local WELT2000 file
development.ini: fix commented mapproxy config path
Merge remote-tracking branch 'tbl/master'