Handle 404s

[arbrandes]

Description

It is perfectly possible for a site to have no default root, or for the user to navigate to a route that doesn't exist.

1. If the user navigates to a route that doesn't exist, they should be shown a 404 page provided by the shell. It is much like the existing ErrorPage, except it is dedicated to 404s.
2. If the user navigates to http://apps.local.openedx.io:8080/ (i.e., the root) on frontend-template-site, they should get redirected to the learner dashboard. To signify that this is an operator decision, the route that implements this should not live in frontend-base, and instead be part of the template-site.

PRs

1. Not found page: feat: add NotFoundPage and route error handling for 404s #194
2. Home redirect: feat: add homeApp to redirect / to the dashboard frontend-template-site#8