Fix an issue with web links

lib/models/post.dart

@@ -74,6 +74,8 @@ class Post implements LikeElement {
 
   bool get canUpdate => access == UserAccessLevels.FULL;
 
+  bool get hasLinkImage => linkImage != null;
+
   bool get canDelete =>
       access == UserAccessLevels.FULL ||
       access == UserAccessLevels.INTERMEDIATE;

lib/ui/tiles/post_tile.dart

@@ -263,10 +263,15 @@ class _PostTileState extends State<PostTile> {
           children: <Widget>[
             Padding(
               padding: const EdgeInsets.only(right: 8.0),
-              child: NetworkImageWidget(
+              child: widget.post.hasLinkImage
+                  ? NetworkImageWidget(
                       url: widget.post.linkImage,
                       width: 70,
                       roundedEdges: false,
+                    )
+                  : Icon(
+                      Icons.link,
+                      size: 70,
                     ),
             ),
             Flexible(