Refresh 12F

[johlym]

It has been a minute or two since meaningful design work took place over in these parts. As a part of a hack-week project, I've taken on giving 12F a facelift.

To name a few changes:

• Yanked the custom CSS and replaced it with Heroku's design library for marketing sites.
• Updated the styling and presentation of a few sections of the site, namely single-page titles and the table of contents.
• Added text citation for each of the articles in APA, MLA, and Chicago format.
• Added YAML support to more easily render the page titles in certain places.
• Added support for safe-inset-area to allow backgrounds to bleed behind device notches and islands, and 2) automatically set a sensible content padding around the edges of a screen (minimum 12px)

I've triggered a review app for folks to be able to interact with this PR.