Whip up a 404 page

Definitely some compromising from the Angular 404 page.

Removes the logged in detection, the fancy display of the URL, etc. Still looks and feels pretty similar.

Biggest contributor to weight is the embedded fonts, which required a CSP change to implement. I cut out the semibold for the links entirely, an I subset the bold to just include the letters we needed for "Page not found". I also subset the main font to just have the English characters. Cut the page size in half with all of that.