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/configuration/camoconfig.js

@@ -0,0 +1,26 @@
+class CamoConfig {
+    constructor(config = { camo: { enabled: false } }) {
+        this.config = config.camo;
+        if (this.config.server) {
+            this.config.server = this.config.server.replace(/\/+$/, '');
+        }
+    }
+
+    isEnabled() {
+        return this.config.enabled;
+    }
+
+    getKey() {
+        return this.config.key;
+    }
+
+    getServer() {
+        return this.config.server;
+    }
+
+    getWhitelistedDomains() {
+        return this.config['whitelisted-domains'] || [];
+    }
+}
+
+export { CamoConfig };