Create database model for calendar selection

[DrumsnChocolate]

This whole thing makes me wonder: is there an easy way to prevent people from having to update their ical links everytime we change a category? Ideally, we would have a situation where we give the user a single link, https://csvalpha.nl/ical/activities?key=privatestuff&user_id=1
And then based on user_id we identify the user and then simply query their calendar settings from our database. We would need to add a place to store this information, but > I think it would be much neater since we would never have to ask a user to update their ical links again!

Originally posted by @DrumsnChocolate in #438 (comment)

This would involve a couple of steps:

1. Design a model structure for keeping track of each user's ical settings
2. Capture the current ical settings people use (by saving the current categories argument people have in their ical links. We can do this, because a sync happens every so often)
3. Maybe we won't be able to capture the info for all members, notify the ones we don't have the info from.
4. Rewrite the current ical endpoint to simply ignore the categories argument, focusing only on the user_id. Simplify the ical link generation
5. Write some frontend components for displaying and editing of a user's ical settings.