Delete old, expired game invites

[Gocnak]

People inviting others to play momentum have the lobby invite stick around as a permanent message, and the bot could automatically delete them as there's no way to "re-open" a closed invite that I know of.

By default delete old ones every 5 minutes? Could change this to every hour or something if the rate limits are bad.

[Hona]

Is this fine to register on message received (99% guarentee, on disconnects the message might be missed)? Otherwise we could get 100% guarantee but have to download x messages from every channel.

It would be simpler logistics wise to download x (100, 200 even?) messages from every channel, but every 30 minutes or so instead of 5 mins.

[Gocnak]

We can do message received, because admins can always delete the stale messages that the bot didn't catch.