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.