Promote Slack chat via badges/buttons on repos

[uniqueg]

I think it would be nice to make the chat more active and allow people to more easily join.

For example, in our repos, we frequently include a chat button/badge that has a persistent invitation to our Slack board (possibly/ideally to a specific channel, like #wes). Something like this (from here):

This currently doesn't seem to be possible for the GA4GH repos (at least not for me), because when I create an invite link, it is only valid for 3 days. And I also didn't find a way of allowing random public users to request an invitation when visiting https://ga4gh.slack.com.