PLT-5897: Respect intrinsic ratio of img in emoji picker (#5987)

Changed style of img emoji picker, also fixed preview of emoji.

webapp/components/emoji_picker/components/emoji_picker_item.jsx

@@ -44,12 +44,14 @@ export default class EmojiPickerItem extends React.Component {
 
         if (this.props.category === 'recent' || this.props.category === 'custom') {
             item =
-                (<span>
+                (<span
+                    onMouseOver={this.handleMouseOver}
+                    onMouseOut={this.handleMouseOut}
+                    onClick={this.handleClick}
+                    className='emoji-picker__item-wrapper'
+                 >
                     <img
                         className='emoji-picker__item emoticon'
-                        onMouseOver={this.handleMouseOver}
-                        onMouseOut={this.handleMouseOut}
-                        onClick={this.handleClick}
                         src={EmojiStore.getEmojiImageUrl(this.props.emoji)}
                     />
                 </span>);

webapp/components/emoji_picker/components/emoji_picker_preview.jsx

@@ -44,7 +44,7 @@ export default class EmojiPickerPreview extends React.Component {
                     <div className='emoji-picker__preview-image-box'>
                         {previewImage}
                     </div>
-                    <div className='emoji-picker__preview-detail-box'>
+                    <div className='emoji-picker__preview-image-label-box'>
                         <span className='emoji-picker__preview-name'>{name}</span>
                         <span
                             className='emoji-picker__preview-aliases'