Update styling of navbar to work on small screen sizes

Changed styling of navbar on smaller and down size screens to put logo centered and menu centered below it.

Added small-down global css variable for smaller screen sizes
On smaller screen sizes increase size of navbar to account for logo above menu
On smaller screen sizes change layout of navbar to flex column so logo will be on top of menu
On smaller screen sizes change menu to be flexbox layout that is centered horizontal and vertical
On smaller screen sizes change margin on first item in menu to have zero margin which removes the margin-right thus knocking the entire menu from being truly centered
On smaller screen sizes change the menu to be 40px from top to allow for logo and to position menu full width on screen by setting left and right to 0 for absolute placement.

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