R1:20ac900e8b6a

Make settings panels more modular and modern

Summary:
Currently, we have a hard-coded list of settings panels. Make them a bit more modular.

- Allow new settings panels to be defined by third-party code (see {D2340}, for example -- @ptarjan).
- This makes the OAuth stuff more flexible for {T887} / {T1536}.
- Reduce the number of hard-coded URIs in various places.

Test Plan: Viewed / edited every option in every panel. Grepped for all references to these URIs.

Reviewers: btrahan, vrana, ptarjan…