Tweak CSS to center-align navbar items in minor-axis direction

[mdoggydog]

In other words:

• When navbar items are laid out horizontally (wide browser window), align them so they are vertically centered.
• When navbar items are laid out vertically (collapsible panel), align them so they are horizontally centered.

[mdoggydog]

This is another one-liner that is easy to add to custom CSS, but I imagine it is beneficial to everyone.

Without this tweak:

• horizontal navbar looks ok... until a larger-than-normal item is introduced;
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• vertical/collapsible navbar is a mess because individual navbar-nav div's are start-aligned.
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With this tweak:

• all items are nicely aligned in either direction.
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