Add InvitationReminderEmailService
What does this MR do?
This MR adds the InvitationReminderEmailService
service to be used by the MemberInvitationReminderEmailsWorker
cron job (!42837 (merged)).
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)
Note1: The implementation of Member#send_invitation_reminder
will be done in another MR (!42854 (merged)).
Note2: Since this feature will be used behind a feature flag, no changelog was added.
Edited by Alex Buijs