Reduce column width on home page

[adborden]

From the first round of usability testing. https://docs.google.com/document/d/1YRlgHC-Venn5oaTY7jrk42OJfabM8171EykkcHC1EQE/edit#

The large column width for the header and helper text on the landing page made it difficult to read text. Some testers also noted that the white text on a background image with white tones made some words difficult to read

Maybe 80 rem?

cc @elinaru

[adborden]

With the new landing page uses our "narrow" layout, 80rem/800px max-width.