Auto-invite members to Slack when they move to Active status

[ChrGriffin]

This MR adds a new controller that redirects to a public Slack invite URL, an email that links to that controller, and it sends that email when a member moves from a list of pre-defined statuses to Active status (for instance, a member who moves from Hiatus to Active is presumably still in the Slack, whereas a member who moves from Applicant to Active is presumably not).

Note: I wanted to simply check if the member's email is in our Slack already, but Slack has that API endpoint locked behind the enterprise paygate.

[ChrGriffin]

PS: did a quick test in production, works exactly as it should!