UX: improve livetrack display to avoid mess on XC days

[jujumo]

Live tracking current status:

Full length track of every LIVE pilots are displayed at once.
See below:

With 100s of pilots, it will quickly becomes unreadable.

Ideas for display:

• completely remove tracks, or
• shorten the track by a constant amount (duration or length)(full on demand)
• eventually, limit length by a user-defined length/duration.
• aggregate pilots by cluster (similar to Leaflet.markercluster)

ideas for more advanced display and seek

If you want to see whats going on, you might want to filter and sort live tracking according to following criteria:

• display only my garden : filter for the area (geoloc or given)
• display only the ones crossing my garden : display only live tracks which crossed the area (geoloc or given).
• what is the longest flight today: sort flights by duration/inverse duration
• what is the biggest flight today: sort flights by distance/inverse distance
• which one can drive me back: filter only specific status (flying, driving)
• find the closest : sort users by distance to geoloc (geoloc or given)
• what is my buddy doing : seek/search a specific name