Update the backlog in preparation for repo walkthrough

[fureigh]

User story

As someone transitioning this project, I want the known remaining work to be clearly represented so that it's actionable, comprehensible and relevant.

Acceptance criteria

• [ ] Issues are clearly segmented by product/area of focus.
• [ ] Any issues that aren't relevant to this phase of the work have been closed.
• [ ] All major known TODOs have been captured.

Implementation notes

• May need to move issues from this GitHub repo to another. If so, update the README accordingly.
• Walkthrough with @alexsoble, who's generously agreed to serve as interim technical point of contact, is currently scheduled for tomorrow.