Separate external assets from esse-dev assets

[sharon-wang]

External assets should be separated from the assets we've created, as they have different licensing and should be clearly identified as external assets.

We should update and organize our assets like we've done in esse-dev/akita#113.

After reorganizing, we may want to update any docs/license files to clearly indicate that we are only licensing our own content under CC and external assets may have different licensing.