Add Rawpixel to authority data as `CURATED`

[zackkrida]

Rawpixel is a curated source of CC0 images, the results seem particularly good. Regardless of result quality, they should likely be classified as a CURATED collection in the https://github.com/WordPress/openverse-api/blob/ad2f74a111af4ea35f939ec597f394c83f6dbc23/ingestion_server/ingestion_server/authority.py code.