[BUG REPORT] Horace warns about channel pins when removing a pin

[debashisbiswas]

Horace warns a channel when the pins approach the pin limit (50). Currently, this reminder is sent both when pinning and unpinning a message. This should be changed such that the reminder is only sent when pinning. Unpinning should not trigger a reminder, as the channel is moving away from the limit, not towards it.