Update landing page UI

[erickzhao]

Closes #36 and #37.

• Removes hero buildings SVG at viewport heights of less than 36rem.
• Adds white background colour to mailchimp input.
• Updates bridge and plateau SVGs

[krubenok]

The scaling doesn't work on a resize but it does on a refresh (at least in safari). I think this is a huge improvement over what we have now and is probably good enough. But I think we should re-open the issue on super low-pri to keep the issue in mind.

[loreina]

I didn't get a chance to review yet... @krubenok