merge certificates.js and nginx.js to reverseproxy.js

when certs change, we have to call into nginx anyway. since they
go hand in hand, just merge those files. modern reverse proxies
do this job integrated already.

src/mail.js

@@ -38,7 +38,6 @@ exports = module.exports = {
 
 var assert = require('assert'),
     async = require('async'),
-    certificates = require('./certificates.js'),
     config = require('./config.js'),
     constants = require('./constants.js'),
     DatabaseError = require('./databaseerror.js'),
@@ -56,6 +55,7 @@ var assert = require('assert'),
     os = require('os'),
     path = require('path'),
     paths = require('./paths.js'),
+    reverseProxy = require('./reverseproxy.js'),
     safe = require('safetydance'),
     shell = require('./shell.js'),
     smtpTransport = require('nodemailer-smtp-transport'),
@@ -524,7 +524,7 @@ function restartMail(callback) {
     const memoryLimit = Math.max((1 + Math.round(os.totalmem()/(1024*1024*1024)/4)) * 128, 256);
 
     // admin and mail share the same certificate
-    certificates.getCertificate({ intrinsicFqdn: config.adminFqdn(), domain: config.adminDomain() }, function (error, cert, key) {
+    reverseProxy.getCertificate({ intrinsicFqdn: config.adminFqdn(), domain: config.adminDomain() }, function (error, cert, key) {
         if (error) return callback(error);
 
         // the setup script copies dhparams.pem to /addons/mail