User unpinning a message does not notify the bot

[col-panic]

I have a message being pinned by the bot. Now it seems like I can't prevent the
user from unpinning this message (do !?) - The real problem is, however, that
if the user unpins it, the bot does not get notified about this.

I tried, and there is simply no update happening.

Same is the case for reactions on messages, but there already exists an issue for this in #1349