Add tracking events for invitation reminders
What does this MR do?
This MR sends a snowplow event for tracking when an invitation is sent and when a user accepts an invitation.
In an effort to split up the work in !41333 (closed) this MR is kept small.
Related Issue: gitlab-org/growth/team-tasks#191 (closed)
Note: Since this feature will be used behind a feature flag, no changelog was added.