Use the admin menu layout for user settings

More ond more settings are available in the user settings. What was once a correct section selector is looking less and less appealing.
In order to solve this issue without restricting the amount of new features we bring in user settings, we should adopt a more complex model that extends more gracefully.

An idea is to use the same layout as the administration menu accordion in the user settings.

See https://forum.xwiki.org/t/shortcut-settings-position/14116 for an instance of discussion about this improvement.