Fix issue with switching moderators while a conversation is ongoing

[tatablack]

If a conversation is paused or stopped by someone different from the person who started it, the duration badges get out of sync (even when a new conversation is then started, the badge on the previous one is also updated).

This has something to do with the timer ID which we set on the main board, but never cancel properly due to the moderator switching (it's also pointless to set it on the board, as the timer - interval in fact - is only started on the original moderator's browser, after all).