Modify app build script.
Squashed 'js/vendor/subtree-leaflet-overlays/' content from commit 89fcbe3

git-subtree-dir: js/vendor/subtree-leaflet-overlays
git-subtree-split: 89fcbe33a5f52c313bb81f79d2a142bb5d7a088b
Merge commit '8b0846dd1a6a9c5a5d50198146501fdc464812fa' as 'js/vendor/subtree-leaflet-overlays'
Add custom esri-leaflet build.
Update leaflet-overlays library.