Add default 404 page
Merge https://git.uwaterloo.ca/science-computing/Scinage/merge_requests/43 first. This PR is based off of it.
When the user accesses an undefined URL (and is expected to receive a "page not found"), he/she will get a stylized 404 page.
This page shows up when the DEBUG setting is set to False (e.g. when we are in the production environment).