lib.xcsoar_.flightpath: don't set threshold to zero

Fixes a bug where the returned flightpath sometimes was cut off
from the front.
collection.js: add 'remove' method
fix-table.js: add 'removeRow' method
flight.js: add sfid attributes to ol features
OpenLayers: fix removal of ProgressiveLineStrings

See https://github.com/TobiasLohner/openlayers/commit/5b08c4814b5821d69ce8beefa97df222d4f40dac
flight.js: add 'removeFlight' method
fix-table.js: add flight removal icon
flight.js: force baro scale update on addFlight and removeFlight

This seems to got broken with bacf581e91543a552e5db36524f8c46119705dc1
Revert "flight.js: force baro scale update on addFlight and removeFlight"

This reverts commit 221734abb25007979805c693bbbb1603cae38829, because
it didn't fix the issue...
baro.js: update time scale if min or max has changed

This is the real fix...
fix-table.js: implicitly unpin a removed flight
commands/flights/*: order by Flight.id

Cosmetics...
fix-table.js: add local variable 'sfid'

D'oh!
model.flight: add FlightPathChunks table to store detailed/splitted flight paths

Will be used for faster PostGIS queries...
model.flight: populate FlightPathChunks table

We add the _full_ flight path to that table. Each row gets 100 points max.

Still needs some filter for path between takeoff/landing only. 100 pts is
a rough guess (250 pts produced slower queries, but smaller tables).
model.flight.FlightPath: add get_near_flights search method

This selects _all_ flights which where near the current flight, but
still uses a large time window. Needs some real distance checks inside
the loop after the query.
model.flight: filter retrieved near flights from database by distance

Prints the detected flights to the console for now...

This could easily be merged with frontend.views.flight._get_near_flights()
model.flight: add meetings table
model.flight_meetings: return a list of meetings, not the query itself
worker.tasks: add find_meetings worker task