Create a UI package for defining relationships in Gutenberg

[srtfisher]

Description

I would like to be able to set the relationships between posts and posts/terms inside of Gutenberg. Right now, the relationship is only able to be created server-side.

This should be a standalone plugin. Relationships should be opt-in, allowing the developer control over defining them in Gutenberg.

Use Case

A user can relate models together in Gutenberg.