chore: update avatar placeholder

lib/screens/gl_todos.dart

@@ -78,8 +78,9 @@ class GlTodosScreen extends StatelessWidget {
                 padding: CommonStyle.padding,
                 child: Row(
                   children: <Widget>[
-                    GitlabAvatar(
-                        url: item.author.avatarUrl, id: item.author.id),
+                    Avatar(
+                        url: item.author.avatarUrl,
+                        linkUrl: '/gitlab/user/${item.author.id}'),
                     SizedBox(width: 12),
                     Expanded(
                       child: Text.rich(

lib/widgets/avatar.dart

@@ -17,6 +17,8 @@ class Avatar extends StatelessWidget {
   final String linkUrl;
   final BorderRadius borderRadius;
 
+  static const fallback = 'images/avatar.png';
+
   Avatar({
     @required this.url,
     this.size = AvatarSize.medium,
@@ -26,7 +28,6 @@ class Avatar extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final fallback = 'images/avatar.png';
     final widget = ClipRRect(
       borderRadius: borderRadius ?? BorderRadius.circular(size / 2),
       child: url == null
@@ -49,41 +50,3 @@ class Avatar extends StatelessWidget {
     );
   }
 }
-
-class GithubAvatar extends StatelessWidget {
-  final String url;
-  final double size;
-  final String login;
-
-  GithubAvatar({
-    @required this.url,
-    this.size = AvatarSize.medium,
-    this.login,
-  });
-
-  @override
-  Widget build(BuildContext context) {
-    return Avatar(url: url, size: size, linkUrl: '/github/$login');
-  }
-}
-
-class GitlabAvatar extends StatelessWidget {
-  final String url;
-  final double size;
-  final int id;
-
-  GitlabAvatar({
-    @required this.url,
-    @required this.id,
-    this.size = AvatarSize.medium,
-  });
-
-  @override
-  Widget build(BuildContext context) {
-    return Avatar(
-      url: url,
-      size: size,
-      linkUrl: '/gitlab/user/$id',
-    );
-  }
-}