Post Upload Progress Notification Improvements

Improves post upload notification by showing upload progress of media items.

Check out `CountingOutputStream` to see how we track the uploaded bytes.

Also fixes a few issues:
 * Fixes notification icon size, #1704
 * Reorganizes code in `PostUploadService`, #1185. But I think I really just put lipstick on a pig ;)

Video of the upload progress: https://cloudup.com/casufV3XXxN
i *love* how the notification shows a thumbnail of the uploading image - very nice touch!