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.

test/test_app.rb

@@ -60,6 +60,18 @@ class AppTest < Minitest::Test
     assert_operator track["properties"]["distance_meters"], :>, 0
   end
 
+  def test_geojson_upload_is_served_back_unchanged
+    content = '{"type":"FeatureCollection","features":[{"type":"Feature","properties":{"kind":"waypoint","name":"Spot"},"geometry":{"type":"Point","coordinates":[17.1,48.1]}}]}'
+    response = post_file("/api/gpx", filename: "spots.geojson", content: content, env: logged_in_env)
+
+    assert_equal 200, response.status
+    assert_equal ["spots.geojson"], json_body(response)["paths"]
+
+    file_response = request.get("/api/gpx/file/spots.geojson")
+    assert_equal 200, file_response.status
+    assert_equal "Spot", json_body(file_response)["features"][0]["properties"]["name"]
+  end
+
   def test_unparseable_gpx_returns_422
     post_file("/api/gpx", filename: "bad.gpx", content: "<gpx><trk unclosed>", env: logged_in_env)