Add NotFound component and 404 redirect

[gabalafou]

Related to https://github.com/conda-incubator/conda-store/issues/899

Description

The basic idea of this pull request is that if the app loads and the user is at some unknown route such as /conda-store/foo/bar, the app will forcefully redirect them to /conda-store/not-found. This redirect should hit the server, which should return this React app along with a 404 HTTP status code. The app will in turn execute at this URL and render a "not found" page.

This pull request:

• Does a client-side, hard redirect to {urlBasename}/not-found if no route matches. By hard, I mean it's not just a state transition via history.pushState() but an actual refresh of the browser page using window.location.replace()
• Adds a NotFound component to be rendered when the {urlBasename}/not-found route matches.

Pull request checklist

• [x] Did you test this change locally?
• n/a Did you update the documentation (if required)?
• [ ] Did you add/update relevant tests for this change (if required)?

[gabalafou]

I made this a draft PR for now until we have server-side support for the {urlBasename}/not-found route.

On the other hand, if we don't mind showing the server-controlled 404 page...

[gabalafou]

Since there is now a companion server pull request, I will take this out of draft.

[gabalafou]

Good catch @peytondmurray, thanks for the review!

[gabalafou]

@peytondmurray, I re-requested your review, could you give this another look when you get a chance?