Converts a URL into a 4-digit ID, similar to bit.ly. Due to the verbose nature of Heroku URLs, they're not much shorter. But... y'know, it's the thought that counts.
User Story: I can pass a URL as a parameter and I will receive a shortened URL in the JSON response.
User Story: If I pass an invalid URL that doesn't follow the valid format, the JSON response will contain an error instead.
User Story: When I visit that shortened URL, it will redirect me to my original link.