[IMP] web_responsive: Set AttachmentViewer z-index upper than chatter

Before this changes when oppening an attachment from the chatter the
attachment was behind the chatter and if we try to hide it, the
attachement disappears.

With this changes the AttachmentViewer appears in front of the chatter
and the problem does not exists anymore.

web_responsive/static/src/components/attachment_viewer/attachment_viewer.scss

@@ -14,7 +14,7 @@
         }
         .o_AttachmentViewer {
             // On-top of navbar
-            z-index: 10;
+            z-index: 1100;
             position: absolute;
             right: 0;
             top: 0;