Add navbar to 404 page

## Feature Request Form
### Problem you are trying to address with this feature

When I land on the not-found page on the site, eg. due to a typo or broken link, it would be useful to have an easy way to find the content that I was trying to get to.

### Your solution

Add the navbar to the 404 page which contains links to the user's profile page, the search box and the user's groups. This was attempted but reverted due to a problem on staging. See https://github.com/hypothesis/h/pull/4276

Start by reviewing the reverted pull request #4276 and identify the implementation repository, 404-page entry point, and navbar components it changed. Confirm how the profile, search, and groups links are exposed, then verify on staging that the navbar appears on a not-found page without repeating the earlier problem.

Written by the indexing model from the issue text.