tag v3.11.0: mailing list subscribe/unsubscribe

A subscribe / unsubscribe toggle in the mailing-list detail toolbar
(MailingListDetailView, which backs Lookup results and ProjectMailingListView),
hidden for lists you own. State comes from membership in the subscriptions
query — MailingList.subscription is a trap that never reports your state — and
the numeric listID the mutations need is read from list(rid:){ id }. Verified
live end-to-end against the ~hutch account.

Unblocks the feature Phase 1 declined, once live testing showed a list is
reachable without a discovery API (Lookup, a project's lists, patchsets).