Use Celery for asynchronous tasks.

Needs a running redis server - see INSTALL.md for short instructions.
views.upload: use async task for in-depth reanalysis of uploaded flights
views.flights: add analyse_all method to reanalyse all flights

Used for debugging atm, should be removed later (there's the
analysis.py script for that purpose).
celery-daemon: add SKYLINES_CONFIG envvar