Don't let a group's members share its mailboxes on

A group's members reach its mailbox through membership, which counts
as owning the account, so every ACL check was skipped: on a scratch
server a member gave an outsider read access to the group's Inbox with
one Mailbox/set shareWith, with no administrator involved and nothing
audited. Who is in a group is an administrator's decision.

AccessToken::is_group_member_only names that case (in the account
only through a group, without Impersonate). For such a member:

- Mailbox/set with a shareWith change, on create or update, is
  refused as forbidden;
- IMAP SETACL and DELETEACL answer NO [NOPERM];
- myRights reports mayShare false, and MYRIGHTS leaves out "a";
  every other right stays.

Administrators and the account itself are unchanged. The JMAP ACL
test's group section now checks all three for a member and that the
outsider still has nothing (specs/multi-account.md, MA-D0, G1).

jmap_tests and imap_tests pass (RocksDB). The IMAP refusal has no test
of its own yet; imap_tests passing shows the rest is unchanged.

tests/src/jmap/mail/acl.rs

@@ -713,6 +713,35 @@ pub async fn test(test: &TestServer) {
             .await,
     );
 
+    // inbuxa: MA-D0: a member can't share the group's mailbox on, and isn't
+    // told it may. Who is in a group is an administrator's decision.
+    assert_forbidden(
+        john_client
+            .set_default_account_id(sales.id_string())
+            .mailbox_update_acl(&inbox_id, bill.id_string(), [ACL::ReadItems])
+            .await,
+    );
+    assert!(
+        !john_client
+            .set_default_account_id(sales.id_string())
+            .mailbox_get(&inbox_id, [mailbox::Property::MyRights].into())
+            .await
+            .unwrap()
+            .unwrap()
+            .my_rights()
+            .unwrap()
+            .acl_list()
+            .contains(&ACL::Administer)
+    );
+    bill_client.refresh_session().await.unwrap();
+    assert!(bill_client.session().account(sales.id_string()).is_none());
+    assert_forbidden(
+        bill_client
+            .set_default_account_id(sales.id_string())
+            .email_get(&email_id, [Property::Subject].into())
+            .await,
+    );
+
     // Remove John from the sales group
     admin
         .registry_update_object(