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.

crates/common/src/auth/access_token.rs

@@ -553,6 +553,16 @@ impl AccessToken {
             || self.inner.access_to.iter().any(|a| a.account_id == account_id)
     }
 
+    /// inbuxa: MA-D0: in the account only because it is a group this token
+    /// belongs to. Such a member has the group's mailbox but may not share it
+    /// on: who is in a group is an administrator's decision, and a share
+    /// would let anyone in.
+    pub fn is_group_member_only(&self, account_id: u32) -> bool {
+        self.inner.account_id != account_id
+            && self.inner.member_of.contains(&account_id)
+            && !self.has_permission(Permission::Impersonate)
+    }
+
     pub fn is_account_id(&self, account_id: u32) -> bool {
         self.inner.account_id == account_id
     }