Add a workflow to handle expired FOSSA invitations

Invitations to join CNCF FOSSA eventually expire (I think it's around 72 hours) so we need a way of re-issuing invitations to Project Maintainers who don't get around to accepting the invitations first time round.

Add a `/fossa-invite resend` command that assigned staff and maintainers can run to get a new inviation sent out to them.

Start by locating the existing `/fossa-invite` command and the FOSSA invitation workflow. Trace how assigned staff and maintainers are authorized, then determine how an expired invitation can be re-issued. Done means the resend command sends a new invitation to eligible users without changing access for others.

Written by the indexing model from the issue text.