[STAR-2714] fix: Don't send reminder email if checklist is completed.
This MR prevents the reminder task from sending an email reminder to users if they have already completed their checklist.
Supporting information
https://tasks.opencraft.com/browse/STAR-2714
Discussions
This MR fixes a previously uncaptured requirement for the feature just merged to allow for multiple reminders.
The message requesting this requirement is here: https://openedx.slack.com/archives/C015KC8CN76/p1660680401453619?thread_ts=1660678913.006759&cid=C015KC8CN76
...But essentially, it remarks that the email should only be sent if the checklist isn't completed.
Testing instructions
Follow the instructions in this MR, except make sure to create at least two people in the subscription, and complete one of the checklists before editing the crontab call to make the emails dispatch.