Restrict access to ACME DNS

Summary:
Only Nasqueron servers can access the Go server.

Anyone can get the homepage.

Ref T1580.

Test Plan:
Tested on Equatower with various Curl from a Nasqueron server
and a workstation to /, /index.html and /register.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1580

Differential Revision: https://devcentral.nasqueron.org/D2179