Consolidate requirements so they can be analyzed

This PR puts all the requirements files into a directory named 'requirements' 

This allows you to use this too requires.io to analyze your requirements and keep track of any packages that or out of date.

Here is the tool running over my fork
https://requires.io/github/Bachmann1234/securedrop/requirements/?branch=highlight-requirements

It highlights several out of date packages. That in itself is not a big deal necessarily. However, the people behind this service track many packages and will provide a different status if there are known security fixes the project is not incorporating.

https://requires.io/security/