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/components/create_post.jsx

@@ -9,6 +9,7 @@ import FilePreview from './file_preview.jsx';
 import PostDeletedModal from './post_deleted_modal.jsx';
 import TutorialTip from './tutorial/tutorial_tip.jsx';
 import EmojiPickerOverlay from 'components/emoji_picker/emoji_picker_overlay.jsx';
+import * as EmojiPicker from 'components/emoji_picker/emoji_picker.jsx';
 
 import AppDispatcher from 'dispatcher/app_dispatcher.jsx';
 import * as GlobalActions from 'actions/global_actions.jsx';
@@ -746,6 +747,7 @@ export default class CreatePost extends React.Component {
                     <span
                         className={'fa fa-smile-o icon--emoji-picker emoji-main'}
                         onClick={this.toggleEmojiPicker}
+                        onMouseOver={EmojiPicker.beginPreloading}
                     />
                 </span>
             );