Follow-up: Update dashboards to homepage in user settings

In #392963 (closed) user settings for dashboard were renamed to homepage.

As a follow-up, let's change the names of values used in Profile preferences for consistency. I think it might be a bit misleading after some time to have items: dashboard_choices.to_json for Homepage preferences.

Check if there are any values/methods outside of preferences_helper.rb that will need a rename in connection to this change.

Example: dashboard_choices to homepage_choices