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/jmap/src/mailbox/get.rs

@@ -14,6 +14,7 @@ use jmap_tools::{Map, Value};
 use std::future::Future;
 use store::ahash::AHashSet;
 use types::{acl::Acl, collection::Collection, keyword::Keyword, special_use::SpecialUse};
+use utils::map::bitmap::Bitmap;
 
 use crate::{api::acl::JmapRights, changes::state::JmapCacheState};
 
@@ -138,6 +139,11 @@ impl MailboxGet for Server {
                             JmapRights::rights::<Mailbox>(
                                 cached_mailbox.acls.as_slice().effective_acl(access_token),
                             )
+                        } else if access_token.is_group_member_only(account_id) {
+                            // inbuxa: MA-D0: everything but sharing it on.
+                            let mut acl = Bitmap::<Acl>::all();
+                            acl.remove(Acl::Share);
+                            JmapRights::rights::<Mailbox>(acl)
                         } else {
                             JmapRights::all_rights::<Mailbox>()
                         }