Add styles for the survey landing page

[chasenlehara]

Please let me know what you think about any part of the landing page and flow for signing up.

Unfortunately the form submission causes the page to load again, so I added some code to scroll down to the success message.

Also, I changed how the Guide navigation item gets highlighted so only the Guides pages will cause it to be active.

There will be a separate PR in the main DoneJS repo with the actual text and other changes.

Here’s what it looks like on desktop:

Here’s what it looks like on mobile: