Adding reminders using pre-existing mailer

[pablo-schmeiser]

Adding options for users to receive reminders via a chosen method an amount of days before their next shift(s).

• Adding new fields to user
• Adding command to send reminders to all users that should be reminded today (you would use this in a cron job or a systemd timer similar to how the recurring shifts work)
• Adding necessary migrations for these changes

I also included a migration, that seemingly was not pushed to the repository (for ease of use)