Create backbone models/views for assets.

cleanup

Updates.

Testing changes.

Testing changes.

asset-index progress state cleanjup.

cleanup
Simplify how we determine if asset is already showing.
Set the URL on the collection instead of the model.
Code review feedback.
Log warning if thumbnail cannot be deleted.
Merge pull request #1031 from edx/christina/backbone-assets

Backbone model/view for asset list.