Add `RelatedSources` Core Data component

[camdendotlol]

Summary

• adds a new component called RelatedSources
  • this component is based heavily on the RelatedItems component, but made more generic to take a sourceType prop and fetch instances, items, or works
• adds an example component and mock routes to Storybook
• adds a deprecated tag to RelatedItems

Notes

• This component does not implement the onClick prop from #285 because the prop depends on changes to RelatedItems that only exist on that branch - whichever PR gets merged second will need to be rebased before merging with the relevant changes made.

Screenshot

[dleadbetter]

Since this reuses so much code from RelatedItems, it might be worth refactoring the code into a RelatedSources component and changing both RelatedItems and RelatedWorks to be thin wrappers over that. But I'm not sure how desirable that might be if the three source types are expected to diverge over time.

I think refactoring both components into RelatedSources (which can also be used for RelatedInstances) is probably the way to go for now. I agreed, we expect the models to diverge at some point in the future, but we're not sure when or even what that will look like. Until then, it will probably be a nicer development experience to only have to make changes to a single component.

For backwards compatibility, let's add a @deprecated annotation to the RelatedItems component and plan to remove it in the next major release.