Custom 404 page added

@ 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