PLT-6659 Fixed upload thumbnails that weren't properly rotated (#6816)

- Used client-side EXIF data to rotate profile picture thumbnails
- Added a small package for correctly translating EXIF orientation into CSS transforms
- Instead of displaying the image using FileReader, used URL.createObjectURL because it is faster
- For upload thumbnails, the original behavior was scaling the entire original image, without accounting for EXIF rotate. I changed this to use the thumbnail image, which does respect rotation.
- The preview image was not available when the upload request returned, because handling the preview image creation was in a goroutine. I used sync.WaitGroup to block until the preview image creation is done.

webapp/package.json

@@ -11,6 +11,7 @@
     "bootstrap-colorpicker": "2.5.1",
     "chart.js": "2.5.0",
     "compass-mixins": "0.12.10",
+    "exif2css": "1.2.0",
     "fastclick": "1.0.6",
     "flux": "3.1.2",
     "font-awesome": "4.7.0",