Per the group chats spec, users under 18 cannot be added to or participate in
group chats. This wires up the client-side half:
- expose isUnder18 flag from the AA context (conservative default when
birthdate is unknown)
- new restrictGroupChatSettings helper that writes
allowGroupInvites: 'none' to the chat actor declaration, called on signup
and on birthdate-update when the user is under 18
- hide the 'New group chat' entry in InitiateChatFlow for under-18 users
- lock the 'Allow group chat invites from' toggle to 'none' for under-18 in
chat settings, with explanatory copy
- pass kind: 'direct' to listConvos for under-18 users so groups don't show
in the requests inbox or count toward the unread/requests badges
Samuel Newman