Formalize and fully modularize settings panel groups

Summary:
Ref T4103. Settings panels are grouped into categories of similar panels (like "Email" or "Sessions and Logs").

Currently, this is done informally, by just grouping and ordering by strings. This won't work well with translations, since it means the ordering is entirely dependent on the language order, so the first settings panel you see might be something irrelvant or confusing. We'd also potentially break third-party stuff by changing strings, but do so in a silent hard-to-detect way.

Provide formal objects and modularize the panel groups completely.

Test Plan: Verified all panels still appear properly and in the same groups and order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16020