MDL-65687 message: increase confirmation dialog zindex.

Make sure that confirmation dialogs (such as block/unblock user,
delete conversation) are shown with prominence in the messaging
drawer.

message/templates/message_drawer_view_conversation_body.mustache

@@ -57,7 +57,7 @@
            </div>
             <div class="hidden text-center p-3" data-region="more-messages-loading-icon-container">{{> core/loading }}</div>
         </div>
-        <div class="p-4 w-100 h-100 hidden position-absolute" data-region="confirm-dialogue-container" style="top: 0; background: rgba(0,0,0,0.3);">
+        <div class="p-4 w-100 h-100 hidden position-absolute z-index-1" data-region="confirm-dialogue-container" style="top: 0; background: rgba(0,0,0,0.3);">
             {{> core_message/message_drawer_view_conversation_body_confirm_dialogue }}
         </div>
         <div class="px-2 pb-2 pt-0" data-region="content-placeholder">

message/templates/message_drawer_view_conversation_footer.mustache

@@ -60,7 +60,7 @@
         {{> core_message/message_drawer_view_conversation_footer_placeholder }}
     </div>
     <div
-        class="hidden position-absolute"
+        class="hidden position-absolute z-index-1"
         data-region="confirm-dialogue-container"
         style="top: -1px; bottom: 0; right: 0; left: 0; background: rgba(0,0,0,0.3);"
     ></div>

message/templates/message_drawer_view_conversation_header.mustache

@@ -47,7 +47,7 @@
         {{> core_message/message_drawer_view_conversation_header_placeholder }}
     </div>
     <div
-        class="hidden position-absolute"
+        class="hidden position-absolute z-index-1"
         data-region="confirm-dialogue-container"
         style="top: 0; bottom: -1px; right: 0; left: 0; background: rgba(0,0,0,0.3);"
     ></div>