Support uploading .geojson files directly

A .geojson file needs no GPX/FIT conversion — it's already the shape
gpx.js expects — so GpxStore#geojson now just reads and returns it
as-is, no cache sidecar. save/tree/the upload dropzone and its zip
handling all pick it up through the existing TRACK_EXTENSIONS list.

Also fixes a bug the new "cached" tree icon would have shipped with:
a GPX/FIT's own .geojson cache sidecar (e.g. "ride.gpx.geojson") also
matches the new .geojson branch in list_children, which would have
listed it as a second, phantom tree entry — excluded explicitly, with
a regression test.

app.rb

@@ -169,8 +169,8 @@ module Maptool
     end
 
     # A .zip (Garmin's bulk/FIT export format) is expanded into its
-    # .gpx/.fit entries instead of being stored itself — nothing downstream
-    # (tree listing, map rendering) knows what to do with a zip.
+    # .gpx/.fit/.geojson entries instead of being stored itself — nothing
+    # downstream (tree listing, map rendering) knows what to do with a zip.
     post "/api/gpx" do
       require_login!
       file = params[:file]