R1:eb673fd783e4

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…