Fix for #39

• Store the menu entries in the settings for explicit and easier management
• This will also check a TODO item from Failed to set up a developmenent environment #80 as we don't need to load the menus in the DB anymore
• Show the menu entries based on the user status
• Get the menu entries from the settings in the header and the sidebar
• Move the categorized menus from the sidebar inside the Categories menu to match with the header
• Move the Review outside the Categories to be easily found by plugin reviewers (should also fix Review the Category menu on small screen #81 )
• Some UI improvements: align them with the global navigation, add icons to the header, move the Admin menu to the sidebar