Add VideoField Support

Addresses #5.

This is currently working, but it's not beautiful.

We try to immediately show the thumbnail (via `thumbnail_small_square` ) but there is some time we need to wait before the video is done processing and thumbnails are ready. We could simply show a placeholder image here or poll the video api endpoint to see if a thumbnail is available.

Video display opens the supplied fulcrumapp.s3.amazonaws.com link in a new tab which seems to work well and keeps us from showing Yet Another Modal.

### TODO:

* [x] Maybe combine `VideoUploader` and `PhotoUploader` into a single `MediaUploader` and subclass out.
* [x] Poll for available video thumbnail or show placeholder image?
* [x] Is opening the video in a new tab acceptable?
* [x] Combine CSS classes `video-row`, `photo-row` become `media-row`. `add-photo`, `add-video` become `add-media`, etc.