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.

test/camo.js

@@ -0,0 +1,54 @@
+const assert = require('assert');
+const Camo = require('../lib/camo');
+const CamoConfig = require('../lib/configuration/camoconfig').CamoConfig;
+
+describe('Camo', () => {
+    const config = new CamoConfig({
+        camo: {
+            server: 'http://localhost:8081',
+            key: '9LKC7708ZHOVRCTLOLE3G2YJ0U1T8F96',
+            'whitelisted-domains': ['def.xyz']
+        }
+    });
+
+    describe('#camoify', () => {
+        it('constructs a camo url', () => {
+            const result = Camo.camoify(config, 'http://abc.xyz/image.jpeg');
+            assert.strictEqual(result, 'http://localhost:8081/a9c295dd7d8dcbc8247dec97ac5d9b4ee8baeb31/687474703a2f2f6162632e78797a2f696d6167652e6a706567');
+        });
+
+        it('bypasses camo for whitelisted domains', () => {
+            const result = Camo.camoify(config, 'http://def.xyz/image.jpeg');
+            assert.strictEqual(result, 'https://def.xyz/image.jpeg');
+        });
+    });
+
+    describe('#transformImgTags', () => {
+        it('transforms an img tag with a src', () => {
+            const attribs = {
+                src: 'http://abc.xyz/image.jpeg',
+                'class': 'some-image'
+            };
+            const expectedAttribs = {
+                src: 'http://localhost:8081/a9c295dd7d8dcbc8247dec97ac5d9b4ee8baeb31/687474703a2f2f6162632e78797a2f696d6167652e6a706567',
+                'class': 'some-image'
+            };
+            const result = Camo.transformImgTags(config, 'img', attribs);
+            assert.deepStrictEqual(result, { tagName: 'img', attribs: expectedAttribs });
+        });
+
+        it('skips img tags with no src', () => {
+            const attribs = { 'class': 'some-image' };
+            const result = Camo.transformImgTags(config, 'img', attribs);
+            assert.deepStrictEqual(result, { tagName: 'img', attribs: attribs });
+        });
+
+        it('fails gracefully', () => {
+            const attribs = { src: 'http://abc.xyz/image.jpeg' };
+            const config = new CamoConfig({ camo: { enabled: true }});
+            config.getKey = () => { throw new Error('something happened'); };
+            const result = Camo.transformImgTags(config, 'img', attribs);
+            assert.deepStrictEqual(result, { tagName: 'img', attribs: attribs });
+        });
+    });
+});

test/configuration/camoconfig.js

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+const assert = require('assert');
+const CamoConfig = require('../../lib/configuration/camoconfig').CamoConfig;
+
+describe('CamoConfig', () => {
+    describe('#constructor', () => {
+        it('strips trailing slashes from the server', () => {
+            const config = new CamoConfig({
+                camo: {
+                    server: 'http://abc.xyz/'
+                }
+            });
+            assert.strictEqual(config.getServer(), 'http://abc.xyz');
+        });
+
+        it('defaults to enabled=false', () => {
+            assert.strictEqual(new CamoConfig().isEnabled(), false);
+        });
+    });
+
+    describe('#getWhitelistedDomains', () => {
+        it('defaults to an empty array', () => {
+            assert.deepStrictEqual(new CamoConfig().getWhitelistedDomains(), []);
+        });
+    });
+});