Invite links expire regularly

[astrojuanlu]

At the moment we're relying on invite links, which have no time limits but expire after 400 signups https://slack.com/intl/en-gb/help/articles/201330256-Invite-new-members-to-your-workspace

On the free, Pro and Business+ subscriptions, members with permission to send invitations can also share an invitation link. Each invitation link can be used by up to 400 people.

This

1. Creates toil, because everytime the link expires, a new one has to be generated and the slack.kedro.org redirect needs to be updated
2. Slack doesn't tell you that a link expired, so it's up to you to monitor it, or someone complaining that the link doesn't work

I recalled there were several auto-invite services but for some reason I can't find them anymore. I did some research and it seems like Slack made some changes to the API and this cannot be easily done anymore.

An alternative could be having some sort of "ping" that checks whether the link still works to send an automated alert. But it's still very reactive.

Another solution could be tracking the number of new users that have joined since the last link was enacted. That way we can preemptively generate a new one when it's about to expire.