ManagerMenu usermanager cannot be removed

[mbarto]

Description

All the menu items of ManegerMenu are configurable to be shown / hidden, except for the UserManager menu. We should refactor the menu as a generic plugin container and allow the single items to be plugins of it.

In case of Bug (otherwise remove this paragraph)

Browser Affected (use this site: https://www.whatsmybrowser.org/ for non expert users)

• [ ] Internet Explorer
• [ ] Chrome
• [ ] Firefox
• [ ] Safari

Browser Version Affected

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Steps to reproduce

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Expected Result

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Current Result

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