Store Terms of Service and other content moderation references for each provider

[zackkrida]

Content on Openverse is only as safe, unbiased, and equitable as the providers we include in the system. We should collect and store more metadata about each provider and how they handle content moderation; what their Terms of Service are, if any, and specifics on how they handle CSAM[^1].

This will help us more easily identify and report these issues upstream to providers in the future, and help make clear to our user the polices in place for every piece of content on Openverse.

[^1]: Child Sexual Abuse Materials (see: https://www.thorn.org/blog/decoding-the-language-of-child-sexual-exploitation-acronyms-to-know/ for more information)