FEATURE BuildTask or QueuedJob to send out reminder email before invitation expires

[muskie9]

Summary

Currently invitations are sent out by an Admin and the invitations have an expiration in days which is set via config on the UserInvitation model. An optional feature that is common place for invitations to sites is reminder emails if someone has not accepted the invitation to register.

Adding a mechanism that can be turned on and scheduled with a cron to send out invitation reminders at various intervals would improve registration success rates while reducing administrative overhead ensuring invitees have access to their invite and ultimately complete registration.