feat(backmatter): Support viewing base64 data

[tuckerzp]

This reworks how we resolve links in backmatter to not always assume data will be in an rlink.

The BackMatterLookup class will make resolving back matter a lot easier in the future.

Testing

http://localhost:3000/system-security-plan/?url=https://raw.githubusercontent.com/EasyDynamics/oscal-demo-content/tuckerzp/base64/system-security-plans/ssp-example.json

The dataflow diagram is in base64

closes #671

[kylelaker]

This is a great start! Thanks for writing tests for this too! Based on a conversation we had earlier, it seems like maybe our current implementation has issues in some browsers, either because of size, corruption in the test data, or corruption as part of the resolution process.

Can you please take a look at seeing if smaller images work or whether re-encoding the document used for testing solves the problem?

I want to keep this in Draft status until we know the implementation actually works, or at least what the specific sharp edges are.

[tuckerzp]

Image was corrupted, I have updated it and things work.

[kylelaker]

Great use of # (though we'll have to refactor it for TS eventually). Good work!

[Bronstrom]

Very slick implementation! This PR taught me about private class members, which I had no idea existed in JS till now.