Simplify changing thumbnails

Changing thumbnails could be done faster.
Currently the workflow looks like that:

1. Click 'Edit' for given video on Uploads page
2. Wait for Edit Item page to load
3. Choose new thumbnail
4. Click Save Changes
5. Wait for Uploads page to load

With some dev changes it could look like:
1. Click thumbnail for gived video on Uploads page
2. Choose new thumbnail from popup/modal window

A lot faster.