[#2] Handle link shortener view submission

When the submit button on the link shortener form is pressed, the data should be handled by the URL controller and saved to the database. If the URL has been shortened before, by the same user, the same URL should be returned. This will prevent spam and allow for the retrieval of lost URLs.

The generated short URL should be in the format of /u/xxxxxx where x is in [a-zA-Z0-9].