Update buttons to accept an optional icon

[craigcook]

Sometimes we want to put an icon on a button, to make it distinct from other buttons or indicate a specific function. For example, a download button might include a little down arrow icon like:

A few approaches we might take:

1. Update button styling to accommodate an inline image, so we could include icons arbitrarily (maybe wrapped in a span, might need a modifier class on the button).
2. Predefine a few specific kinds of icon buttons for specific purposes, then a theme class could handle the icon in CSS (a pseudo-element with a background).

Additional thoughts:

• We might need style alternatives for icon-on-left or icon-on-right, unless we decide to commit to only one.
• Icon position should respect language direction.
• We probably shouldn't allow doubling icons (no left AND right). Unless there's a very good use case for two icons in a button?
• If we use Protocol icon assets we'll need them in both dark and light (though if it's an inline SVG we could color the icon with CSS)