Refresh the emoji library (#7001)

* refresh the emoji library

* fix img_trans.gif

* compress sprite sheet

* remove note on compression - webpack seems to handle compression decently

* better emoji sheet preloading

* requested changes

webapp/webpack.config.js

@@ -318,6 +318,7 @@ if (TEST) {
     config.plugins.push(
         new CopyWebpackPlugin([
             {from: 'images/emoji', to: 'emoji'},
+            {from: 'images/img_trans.gif', to: 'images'},
             {from: 'images/logo-email.png', to: 'images'},
             {from: 'images/circles.png', to: 'images'},
             {from: 'images/favicon', to: 'images/favicon'},