add_ui_sidebar_advanced: Add UI Elements to the Sidebar (Advanced Tab)
Description
This function registers UI elements to the secondary (rear-most) tab
on the dashboard sidebar. The default name of the tab is Advanced
but can be renamed using the tabname argument.
whether to append the elementlist to the currently
registered elements or replace the currently registered elements completely
tabname
change the label on the UI tab (default = "Advanced")
Shiny Usage
Call this function after creating elements in program/ui_sidebar.R to register
them to the application framework and show them on the Advanced tab in the
dashboard sidebar