feat: Certain header links should be represented as buttons, or with icons

The comp shows that the "download" and "donate" CTAs to the right of the top nav are not simple text links. The former leverages a custom "download" icon and the latter is shown with a Bootstrap .btn-warning style applied.

This MR seeks to address this difference, and this commit is the initial work done against that need, providing a first pass at the HTML, CSS and SVG used to get there. (Browser and platform testing to come.) Main nav collapse-to-stacked breakpoint has been adjusted to accomodate width of buttons.

Closes #57 (closed).