Replace Slack invite link (#1284)
Replace Slack invite link (#1284)

This new link will never expire.
google/trillian,Martin2112/trillian,AlCutter/trillian,daviddrysdale/trillian,gdbelvin/trillian,AlCutter/trillian,RJPercival/trillian,daviddrysdale/trillian,gdbelvin/trillian,google/trillian,daviddrysdale/trillian,RJPercival/trillian,Martin2112/trillian,gdbelvin/trillian,Martin2112/trillian