Add "reminders" field to user model

[fisher60]

To accommodate persistent reminders on Friendo bot, we need users to be able to store multiple reminders. This is probably best done via a foreign key.

Reminders shall contain the datetime of the reminder (so that it may be served in ISO format through the API), the content of the reminder, the reminder ID (probably ok to use django's default ID's), and the owner/owner's discord ID (through the use of the foreign key).
It may also be useful to include a "creation_time" field, or other meta data that seems useful for metrics or using the reminder features.