Migrate `reminders` endpoint from the site API

[D0rs4n]

At this point, I believe it's time we started migrating a few of the endpoints, there is an issue regarding the off-topic names endpoint, however I think we can also migrate the reminders endpoint, to see a more complicated one in this new environment as well.
(Although, the Alembic migration environment hasn't been set up just yet, a PR should come up anytime now.)
I would be glad If I could implement( or more like migrate) this one myself. (If it's possible, of course)