core: forward compatible support for owners/admins/moderations deleting channel and public group messages without limitations (#6962)

* plan: delete channel messages

* core: allow deletion of own messages in public groups in channels to moderators-owners

---------

Co-authored-by: Evgeny @ SimpleX Chat <259188159+evgeny-simplex@users.noreply.github.com>

src/Simplex/Chat/Library/Subscriber.hs

@@ -2165,7 +2165,7 @@ processAgentMessageConn vr user@User {userId} corrId agentConnId agentMessage =
             in case sndMemberId_ of
               -- regular deletion
               Nothing
-                | sameMemberId memberId mem && rcvItemDeletable ci brokerTs ->
+                | sameMemberId memberId mem && (publicGroupItemDeletable mem || rcvItemDeletable ci brokerTs) ->
                     delete cci False Nothing
                 | otherwise ->
                     messageError "x.msg.del: member attempted invalid message delete" $> Nothing
@@ -2201,6 +2201,7 @@ processAgentMessageConn vr user@User {userId} corrId agentConnId agentMessage =
             messageError ("x.msg.del: channel message not found, " <> tshow e) $> Nothing
       where
         isOwner = maybe True (\m -> memberRole' m == GROwner) m_
+        publicGroupItemDeletable mem = useRelays' gInfo && memberRole' mem >= GRModerator
         RcvMessage {msgId} = rcvMsg
         findItem = do
           let tryMemberLookup mId =