Custom 404 page added

[ihsaan-ullah]

@ mention of reviewers

@Didayolo @bbearce @dtuantran

A brief description of the purpose of the changes contained in this PR.

Custom 404 page addded

Screenshot

A checklist for hand testing

• use any invalid URL to check e.g. https://www.codabench.org/aaa/

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

Fixing #700

[Didayolo]

@ihsaan-ullah Thank you very much for taking care of this!

• I have changed the "return home" URL from codabench.org to /, to avoid local instances and other deployments of the platform to return to the production instance

• I have fixed flake8 formatting

• I removed the changes in src/urls.py. What was the purpose of it? It just created a string variable, not using it:

from django.conf.urls import handler404
handler404 = "pages.views.page_not_found_view"