Adding parenthesizes for the @navbarCollapseDesktopWidth variable declaration

This commit forces the js Less script (less-1.4.1.min.js) to evaluate the expression in media selectors like "@media (max-width: @navbarCollapseDesktopWidth)".
Without the parenthesizes, this expression is evaluated as "@media (max-width: 979px + 1)" instead of "@media (max-width: 980px)".

Best regards