Add thumbnails for media resources #29
Description
Each media resource should have a thumbnail. This is especially important for features such as an image gallery where many images are being displayed at once.
Acceptance Tests
1. Uploading Images
After uploading an image, a thumbnail should be created which is at most 600 by 600 pixels.
2. YouTube Videos
YouTube videos should have their thumbnails pulled from the YouTube API.
3. Replacing Images
If a media resource has a new image uploaded to it, the thumbnail should be replaced.