Add data-avatar-of="@..." to all user avatars

app/javascript/mastodon/components/avatar.js

@@ -64,6 +64,7 @@ export default class Avatar extends React.PureComponent {
         onMouseEnter={this.handleMouseEnter}
         onMouseLeave={this.handleMouseLeave}
         style={style}
+        data-avatar-of={`@${account.get('acct')}`}
       />
     );
   }

app/javascript/mastodon/components/avatar_overlay.js

@@ -21,8 +21,8 @@ export default class AvatarOverlay extends React.PureComponent {
 
     return (
       <div className='account__avatar-overlay'>
-        <div className='account__avatar-overlay-base' style={baseStyle} />
-        <div className='account__avatar-overlay-overlay' style={overlayStyle} />
+        <div className='account__avatar-overlay-base' style={baseStyle} data-avatar-of={`@${account.get('acct')}`} />
+        <div className='account__avatar-overlay-overlay' style={overlayStyle} data-avatar-of={`@${friend.get('acct')}`} />
       </div>
     );
   }