Don't let a group's members share its calendars, address books or files

#146 stopped a group's members sharing its mailboxes on. The same
shortcut lets them through everywhere else a group owns things: a
member counts as the account's owner, so Calendar/set, AddressBook/set
and FileNode/set skip the share check, and so does the WebDAV ACL
method. Who has what a group owns is decided by who is in the group.

For a member through a group only (is_group_member_only):

- Calendar/set, AddressBook/set and FileNode/set refuse a shareWith
  change as forbidden, on create and update; for files at the top of
  the account too, not only inside a folder;
- the DAV ACL method answers 403 on the group's calendars, address
  books and files;
- myRights reports mayShare false (JmapRights::owner_rights), and the
  DAV current-user-privilege-set leaves out all and write-acl.

Reading who something is shared with is unchanged, as in JMAP.

Tests: a new jmap::group_share module has a member create with a
share, create without one (and check myRights), share afterwards, and
an outsider reach each kind; the WebDAV ACL test has a member try the
ACL method on the group's folders; the IMAP ACL test now checks #146's
SETACL refusal, which had no test of its own. jmap_tests, webdav_tests
and imap_tests pass (RocksDB). specs/multi-account.md MA-D0.

crates/jmap/src/file/set.rs

@@ -250,6 +250,16 @@ impl FileNodeSet for Server {
                 },
             };
 
+            // inbuxa: MA-D0: a group's members don't share what it owns on,
+            // at the top of its files as anywhere else
+            if has_acl_changes && access_token.is_group_member_only(account_id) {
+                response.not_created.append(
+                    id,
+                    SetError::forbidden().with_description("This belongs to a group. Only an administrator can change who has it."),
+                );
+                continue 'create;
+            }
+
             // Inherit ACLs from parent
             if file_node.parent_id > 0 {
                 let parent_id = file_node.parent_id - 1;
@@ -509,6 +519,14 @@ impl FileNodeSet for Server {
                     continue 'update;
                 }
             }
+            // inbuxa: MA-D0: a group's members don't share what it owns on.
+            if has_acl_changes && access_token.is_group_member_only(account_id) {
+                response.not_updated.append(
+                    id,
+                    SetError::forbidden().with_description("This belongs to a group. Only an administrator can change who has it."),
+                );
+                continue 'update;
+            }
             if has_acl_changes {
                 if let Err(err) = self.acl_validate(account_id, &new_file_node.acls).await {
                     response.not_updated.append(id, err.into());