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/dav/src/common/acl.rs

@@ -133,6 +133,10 @@ impl DavAclHandler for Server {
         {
             return Err(DavError::Code(StatusCode::FORBIDDEN));
         }
+        // inbuxa: MA-D0: a group's members don't share what it owns on.
+        if access_token.is_group_member_only(account_id) {
+            return Err(DavError::Code(StatusCode::FORBIDDEN));
+        }
 
         // Validate ACEs
         let grants = self
@@ -565,7 +569,13 @@ impl Privileges for AccessToken {
         grants: &ArchivedVec<ArchivedAclGrant>,
         is_calendar: bool,
     ) -> Vec<Privilege> {
-        if self.is_member(account_id) {
+        if self.is_group_member_only(account_id) {
+            // inbuxa: MA-D0: everything but sharing it on.
+            Privilege::all(is_calendar)
+                .into_iter()
+                .filter(|privilege| !matches!(privilege, Privilege::All | Privilege::WriteAcl))
+                .collect()
+        } else if self.is_member(account_id) {
             Privilege::all(is_calendar)
         } else {
             current_user_privilege_set(grants.effective_acl(self))