Stripe CTF3: add level4
Stripe CTF3: link to @henrik-muehe’s write-up
Stripe CTF3: link to AbiusX’s write-up
Stripe CTF3: link to @ajtulloch’s write-up

Closes #21.
Stripe CTF3: link to @rrjamie’s write-up

Closes #22.
Stripe CTF3: link to @xthexder’s solutions

Closes #23.
Stripe CTF3: link to @janosgyerik’s test runners

Closes #24.
Stripe CTF3: link to @vinzenz’s solutions

Closes #25.
Stripe CTF3: link to @kratorius’ write-ups

Closes #26.
Stripe CTF3: use consistent formatting