Reorganize team page by working group

Restructure the Meet the Team page around React's working groups and
Leadership Council, and refresh membership.

- Add a Leadership Council section (working-group representatives) and a
  Working Group members section, replacing the single flat team list.
- Tag each member with their working groups via a new group prop on
  TeamMember, with a star marking Leadership Council representation. For
  representatives, their represented group is listed first.
- Add an Advisors section for people who support React outside the
  technical working groups (React Foundation, security reviews, etc.).
- Move former members who are no longer active to Emeritus.
- Add new members with GitHub profile photos; fall back to a neutral
  placeholder avatar where no photo is available.
- Make the TeamMember bio optional so members without a bio render
  cleanly.

Author: mattcarrollcode