Add ability to enroll certain users into the new navigation

What does this MR do and why?

Adds the ability to enroll certain users into the new navigation. With this flag we are able to set the default toggle state to "on" for a user. These users will see the new navigation automatically, but they will still be able to disable it via the usual toggle.

How to set up and validate locally

First, either create a brand new user, or prepare your root user like this in the console:

Feature.enable(:super_sidebar_nav) # if you haven't done that already earlier

user = User.first
user.update(use_new_navigation: nil) # like you never touched the toggle
Feature.enable(:super_sidebar_nav_by_default, user)

• Open your GDK in the browser now, log in, and you should see the new navigation.
• Now use the toggle to disable the new nav.
• Now enable it again.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.