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

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3257