Nginx configuration for www.dereckson.be and deploy staging area

Summary:

• deploy www51.dereckson.be files as a staging area on the devserver
• configure nginx to serve both domains on the legacy webserver

This is a follow-up for 88ad7d6558ab.

Test Plan:
Browse https://www51.dereckson.be

No-op for www part.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1912