Add custom pages for 404 and 500

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🔍 Inspect: https://zeit.co/opentechschool/websitenext/8gc9qkqfn ✅ Preview: https://websitenext-git-404-page.opentechschool.now.sh

[xMartin]

Nice! It works even with sending a 404 from the server already. Probably thanks to Zeit understanding Next.js.

Could merge this improvement already and open another PR for the design.

[Twissi]

Added squid to the 404 page

[gsambrotta]

👏 👏 👏 Beautiful squid @Twissi