Detect when patterns are using self-hosted images/media

[ryelle]

If a pattern is created on a 3rd party site, the pattern author isn't restricted to the Openverse CC0 search, and can upload any media. When they copy it over to the pattern creator, it's still using the media from their site, and anyone who copies that pattern once submitted will also be loading it from the pattern author's site. Additionally, we have no information about the image, so no way of knowing whether it's CC0.

We should detect this somehow when content is pasted into the pattern creator, or in the validation step.

The same check will also need to be run on existing patterns, as there are a few already doing this. Maybe reach out to specific pattern authors if it seems to be consistent.

[ryelle]

Maybe using the id as mentioned in #698 would help— blocks with an ID should get some kind of alert/blocked submission? If they replace the image with an openverse image, it should clear the ID.