Support proxying chat images via camo

Camo: https://github.com/atmos/camo.  This has a couple advantages over
just allowing images to be dumped as-is:

  - Prevents mixed-content warnings by allowing the server to proxy HTTP
    images to an HTTPS camo instance
  - Protects users' privacy by not exposing their browser directly to
    the image host
  - Allows the camo proxy to intercept and reject bad image sources
    (URLs that are not actually images, gigapixel-sized images likely to
    DoS users' browsers, etc.)

Whitelisting specific domains is supported for cases where the source is
known to be trustworthy.

src/channel/chat.js

@@ -6,6 +6,7 @@ var util = require("../utilities");
 var Flags = require("../flags");
 var url = require("url");
 var counters = require("../counters");
+import { transformImgTags } from '../camo';
 
 const SHADOW_TAG = "[shadow]";
 const LINK = /(\w+:\/\/(?:[^:\/\[\]\s]+|\[[0-9a-f:]+\])(?::\d+)?(?:\/[^\/\s]*)*)/ig;
@@ -381,7 +382,17 @@ ChatModule.prototype.filterMessage = function (msg) {
         }
     });
 
-    return XSS.sanitizeHTML(result);
+    let settings = {};
+    const camoConfig = Config.getCamoConfig();
+    if (camoConfig.isEnabled()) {
+        settings = {
+            transformTags: {
+                img: transformImgTags.bind(null, camoConfig)
+            }
+        };
+    }
+
+    return XSS.sanitizeHTML(result, settings);
 };
 
 ChatModule.prototype.sendModMessage = function (msg, minrank) {