Fix minor web bugs

lib/components/avatar.dart

@@ -42,7 +42,7 @@ class Avatar extends StatelessWidget {
         backgroundImage: mxContent.mxc?.isNotEmpty ?? false
             ? AdvancedNetworkImage(
                 src,
-                useDiskCache: true,
+                useDiskCache: !kIsWeb,
               )
             : null,
         backgroundColor: mxContent.mxc.isEmpty

lib/components/content_banner.dart

@@ -53,7 +53,7 @@ class ContentBanner extends StatelessWidget {
                           fit: BoxFit.cover,
                           image: AdvancedNetworkImage(
                             src,
-                            useDiskCache: true,
+                            useDiskCache: !kIsWeb,
                           ),
                         )
                       : Icon(defaultIcon, size: 300)

lib/utils/matrix_file_extension.dart

@@ -18,7 +18,9 @@ extension MatrixFileExtension on MatrixFile {
       var element = html.document.createElement('a');
       element.setAttribute(
           'href', html.Url.createObjectUrlFromBlob(html.Blob([bytes])));
-      element.setAttribute('target', "new");
+      element.setAttribute('target', "_blank");
+      element.setAttribute('rel', "noopener");
+      element.setAttribute('download', fileName);
       element.setAttribute('type', mimeType);
       element.style.display = 'none';
       html.document.body.append(element);