In-line reminders in places where future dates are available

• Add reminders to activity dates and course start/end dates.

This could connect with MOBILE-3908 although it won't necessarily create a calendar reminder (maybe is not necessary? We can just add quick reminders, not events like Google does)

Shaping:

• On Dates of courses (course info) and activities (module info) when a date is future, a button with a bell will be shown next to it.
• When clicking the button once (if there's no reminder set) it will set a new reminder (only once is permitted per date) for the same day and time as the date declarates.
• If previous reminder is already set, a menu will be shown with some suggestions of time to be set, and an advanced custom settting. Also you will be able to delete the reminder from there.
• When a reminder is set, the bell icon changes colour and becomes active (check Netflix, Reddit, Youtube, Twitch).
• Notifications are based on time before, so when the teacher edits a date and the user already had a reminder set time will be recalculated.
• UX prototype: https://www.figma.com/file/M75foZlqCqK47MMBXda88T/Moodle-App-4.1-Features-%26-Exploration?node-id=80%3A36899

Discarded follow-up solutions (see epic link)

• See the past/future reminders in the notifications tab. And be able to set and delete.
• See the reminders set on the calendar.