Feat: Support DFIQ objects

[tomchop]

This PR adds support for DFIQ objects as specified in https://dfiq.org

Some notes:

• The source of truth here is the object YAML, any object creation or update comes from a full DFIQ yaml, which is then stored. This makes it easy to preserve the original format and also to "dump" DFIQ objects to their YAML equivalent.
• Parent / child relationships are enforced through the web API only. It's possible to directly create DFIQ objects in the database without enforcing this
  • e.g., it's possible to directly create a DFIQ object with parent IDs that do not match an object in the database, but not possible through the API)
• Corresponding frontend PR: Feat: Support DFIQ objects yeti-feeds-frontend#140

Tasks

• Base scaffolding
• Tests
• API from_yaml endpoint
• API patch updates parent links
• Tests (API + schema)
• from_yaml function
• to_yaml function

[udgover]

I reviewed this PR by directly using the feed and the UI and everything is looking well. I also had a look at the codebase and nothing hurts me. :)

Let's go!