Switch the sandbox domain to an explicit choice on the users part

This allows subdomains or separate domains - a convenience for setups that have a wildcard pointing at the CC server.

.env.sample

@@ -1,7 +1,11 @@
 TYPE=cryptpad
 
+
 DOMAIN=cryptpad.example.com
 
+# This is a separate domain for the secure side of Cryptpad. It can be any other domain (subdomain or separate domain)
+SANDBOX_DOMAIN=sandbox.cryptpad.example.com
+
 ## Domain aliases
 #EXTRA_DOMAINS=', `www.cryptpad.example.com`'
 LETS_ENCRYPT_ENV=production