Adds some validation to new user registration.

backend/requests/register.sh

@@ -1 +1 @@
-curl -X POST -H "Content-Type: application/json" -d '{"username": "a", "email": "b", "password": "c", "password_repeat": "d"}' localhost:8000/api/register
+curl -X POST -H "Content-Type: application/json" -d '{"username": "roflin", "email": "user@example.com", "password": "oreokoekje123", "password_repeat": "oreokoekje123"}' localhost:8000/api/register