Allow users to remove shortened URLs

[mikybars]

Problem

Currently there is no way of editing or removing shortened URLs so that they point to a different place. Therefore users that create URLs with meaningful names (aka custom paths, e.g. migueli.to/dotfiles) are stuck with their initial choice till the end of times.

Concerns

In order to support the remove feature we actually require the notion of users first so that a given user can not remove shortened URLs created by others. In addition, provided we implement users we might as well restrict custom paths only to logged in users to prevent "URL hoarding attacks".

Tasks

• Implement user management via Cognito.
• Map the Cognito configuration to the Serverless framework format.
• Implement a new Lambda function to list all the shortened URLs created by a user.
• Implement a new Lambda function to remove a URL.