Update 404 handling

[davemcorwin]

User Story

A user should see the Federalist 404 content and not be redirected to a 404 page

Background (Optional)

We shouldn't redirect on 404s, we should just return the 404 page and status code.

https://gsa-tts.slack.com/archives/G7EFTFW03/p1632783010005600

function fourOhFourhandler(req, res) {
  res.status(404).render('404.njk')
}

Acceptance Criteria

• [x] visiting a non-existing page renders 404 content
• [x] Change made live via Federalit

After evaluating, edit this part:

Level of effort - <low/medium/high>

Implementation outline (if higher than "low" effort):

• [ ] <TODO>