Improve welcome message cooldown criteria

Currently a welcome message will not be sent after induction if there has been a previous welcome message sent within the past 7 messages of the channel. This prevents frequent spam of welcome messages.

I suggest that the cooldown condition also includes a time based criteria. That is that the welcome message should be allowed to be sent if the previous condition was not met, but it has been greater than a certain amount of cooldown time. So a new welcome message can be sent right after a previous welcome message, so long as enough time has passed since the first welcome message was sent.

Thoughts on whether to include this change would be appreciated.