add scroll option to box

[tbradley1013]

This PR allows for users to create a scroll-able box if they specify the height. The scroll bar is created if height is defined and scroll = TRUE and scrolling is enabled if the height of the content exceeds the height of the box.

While the scroll bar can be added manually to the box class in a css file, it can only be added to all boxes as a result. Users are unable to change or add to the class of the top div in the box output. Specifying the class argument in the box function appends to the class of the inner div which does not look as good for the scroll bar. This function will add the style and the class to the top div if the height is specified also.

I looked at adding it to tabBox for consistency but it seems more practical to allow to the user to just define the class and/or style to tabPanel as setting class or style in that function access the top div of the tab.

If there is anything else I should do to help get this merged, please let me know!

Thanks!

[CLAassistant]

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