Closes #95 Create 404 Page

[bukajsytlos]

Closes #95 Create 404 Page

[Brutus5000]

Just a few words on the state as discussed with bukajsytlos:

1. We need dedicated 404 pages for html and for application/json
2. We first want to check if we can serve a unified 404 page via our reverse proxy.
3. Depending on traefiks capabilities we might need to implement the missing solution in the api.