Create a `Thumbnail` model

[tcely]

This requires the Codec data to be populated:

• Add codecs for the thumbnail asset type #1458

Orientation is one of:

• landscape
• portrait

Thumbnail table partial example:

extension	Codec	min_height	max_height	resized_height	min_width	max_width	resized_width	orientation	resized
jpg	[Codec.pk of <thumbnail,jpg>]	360	1080	NULL	480	1920	NULL	landscape	False
webp	[Codec.pk of <thumbnail,webp>]	360	1080	720	480	1920	1280	landscape	False

[snakefood3232]

This requires the Codec data from #1458 to be populated first. I'd handle this by creating the Thumbnail model with the specified fields and orientation choices, linking it to the Codec table. Can have a PR up in a day after the prerequisite is merged. I've implemented similar media metadata models in Django projects before.

[tcely]

I look forward to seeing your pull request.