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.

views/index.erb

@@ -48,12 +48,12 @@
     <strong>Tracks</strong>
     <div class="gpx-panel__actions">
       <button type="button" class="gpx-panel__action" data-action="new-folder" title="New folder" aria-label="New folder"></button>
-      <button type="button" class="gpx-panel__action" data-action="upload" title="Upload GPX/FIT file" aria-label="Upload GPX/FIT file"></button>
+      <button type="button" class="gpx-panel__action" data-action="upload" title="Upload GPX/FIT/GeoJSON file" aria-label="Upload GPX/FIT/GeoJSON file"></button>
       <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
     </div>
   </div>
 
-  <input type="file" class="gpx-file-input" accept=".gpx,.fit,.zip" multiple hidden>
+  <input type="file" class="gpx-file-input" accept=".gpx,.fit,.geojson,.zip" multiple hidden>
   <p class="gpx-upload__status" hidden></p>
 
   <div class="gpx-tree"></div>