Add option for gravatar url

config-example.json

@@ -113,6 +113,7 @@
   "rabbitMQ": "amqp://localhost/",
   "email_jwt_secret": "@EMAIL_JWT_SECRET@",
   "google_analytics": "UA-XXXXXXXX-X",
+  "gravatar_url": "https://gravatar.loli.net/avatar",
   "cdnjs_url": "https://cdnjs.loli.net/ajax/libs",
   "self_cdn_url": "",
   "google_fonts": {

utility.js

@@ -142,7 +142,7 @@ module.exports = {
     });
   },
   gravatar(email, size) {
-    return gravatar.url(email, { s: size, d: 'mm' }).replace('www.gravatar.com', 'gravatar.loli.net');
+    return gravatar.url(email, { s: size, d: 'mm' }).replace('//www.gravatar.com/avatar', syzoj.config.gravatar_url);
   },
   async parseTestdata(dir, submitAnswer) {
     if (!await syzoj.utils.isDir(dir)) return null;