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/api/acl.rs

@@ -187,6 +187,21 @@ impl JmapRights {
         Value::Object(obj)
     }
 
+    /// inbuxa: MA-D0: an owner's rights, which for a group's member are
+    /// everything but sharing it on.
+    pub fn owner_rights<T: JmapSharedObject>(
+        access_token: &AccessToken,
+        account_id: u32,
+    ) -> Value<'static, T::Property, T::Element> {
+        if access_token.is_group_member_only(account_id) {
+            let mut acl = Bitmap::<Acl>::all();
+            acl.remove(Acl::Share);
+            Self::rights::<T>(acl)
+        } else {
+            Self::all_rights::<T>()
+        }
+    }
+
     pub fn rights<T: JmapSharedObject>(
         acls: Bitmap<Acl>,
     ) -> Value<'static, T::Property, T::Element> {