Add an option to hide image previews

Applies to images, stickers, and URL previews.

Fixes https://github.com/vector-im/riot-web/issues/10735

res/css/views/messages/_MImageBody.scss

@@ -19,6 +19,21 @@ limitations under the License.
     margin-right: 34px;
 }
 
+.mx_MImageBody_hidden {
+    position: absolute;
+    left: 0;
+    top: 0;
+    bottom: 0;
+    right: 0;
+    text-align: center;
+    border: 1px dashed $input-border-color;
+
+    // To center the text in the middle of the frame
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+
 .mx_MImageBody_thumbnail {
     position: absolute;
     width: 100%;

res/css/views/messages/_MStickerBody.scss

@@ -22,3 +22,14 @@ limitations under the License.
     position: absolute;
     top: 50%;
 }
+
+.mx_MStickerBody_hidden {
+    max-width: 220px;
+    text-decoration: none;
+    text-align: center;
+
+    // To center the text in the middle of the frame
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}