User roles form: Replace the "Required" column with a useful "Description" column

This is a follow-up to #3402, specifically for the user roles form.

@klonos

I do not think that a separate "required" column provides any value. This status will never be removed for the anonymous and authenticated roles, and it will never be assigned to any of the other roles. I would much better prefer that we had a pseudo-description column in its place ("pseudo" because we do not actually provide any means for descriptions to be added to roles), explaining why these two roles are special and can't be removed.

Here's what I had in mind:

On mobile, it would look like this:

PS: The more/less toggle makes this part of #5090.