More resilient user settings

[OhKai]

Describe the bug
If the user already has some user settings stored in their LocalStorage, any new default values won't be added. This leads to bugs on the site where some values are expected to exist in the settings store.

To Reproduce
Add a new default setting to lib/user-preferences.ts without deleting the LocalStorage and try to read it in a component.

Expected behavior
The website should always work, regardless if the user visited a long time ago and has very outdated LocalStorage data.
Ideally, even old settings should be migrated and not simply overwriten.

Additional context
There needs to be some migration code whenever we structurally change the settings. E.g. make sure to add new settings once and change structure of existing settings if types/props change.