Visual bugfixes for mobile

[katiestahl]

          Looking great overall, this is awesome. A few thoughts that could be addressed here or broken out to separate issues

• I think the logo on the top bar should downsize a bit when the menu switches to the hamburger. That would reduce the height of the top bar substantially which gives us more screen real estate
• The "current contributors" section in the About page could probably collapse to one column once we're in mobile-width territory
• I think it might look cleaner to make the API/downloads/GitHub links go vertically-stacked when width is minimal. Otherwise they look slightly offcenter from the text above:

• It would be great if we could make this indentation flush with the header (also would be nice if the bottom of the download button was vertically aligned with the bottom of the number text:

Originally posted by @jsstevenson in #469 (comment)