Feature: Provide Slack integration for when Apple breaks boarding

[kdawgwilk]

So recently Apple made some changes to their TestFlight API which caused boarding to be unable to add testers which caused us some pretty big problems cause we are in the middle of launching a fairly large beta. What we ended up doing was fork boarding and add a redirect to a google form where they could put in their info and that would get added to a Google sheet. We then add an integration that watched the google sheet and posted to slack with new rows that came in for new beta sign ups and we would manually go and add the users in iTunesConnect. It would fantastic if we could provide a Slack URL to boarding through an ENV variable that boarding would fallback to if the api calls failed and posted a message to slack with the user info so we could get them added manually if the api call failed and there was no Slack URL it would just display an error message like it currently does. If the Slack URL exists it would change the wording in the success page to say that an invite will be sent out "sometime in the future" since it won't be immediate due to it being a manual process.