Top nav improvements

[Erioldoesdesign]

So the current global nav is awesome, but we could make it even more awesome 😄

When a user is at the top of the fossresponders page, the nav has an underline in brand blue #58B2E4 5px high.

As the user scrolls, the top nav is sticky.

As the user scrolls the foss responders logo (https://drive.google.com/open?id=1xPxmLTwacQ5BiSLecF2EI4I7jfijQMeL) aligns to the left side of the nav bar.

As the user scrolls or clicks on a nav item, that current section gets an 'arrow' pointing up to that section in the navbar and the text becomes bold and changes to the brand darker blue #2292CE :

[RichardLitt]

Stale. Thank you, @Erioldoesdesign. Feel free to reopen if you feel it is necessary.