[PLT-6744] Add "Only visible to you" note for ephemeral messages (#6790)

webapp/sass/layout/_post.scss

@@ -1383,6 +1383,12 @@
         }
     }
 
+    .post__visibility {
+        @include opacity(.6);
+        font-size: .9em;
+        padding: 8px;
+    }
+
     .post__time {
         @include opacity(.6);
         font-size: .9em;