[IMP] website: Display message to drag&drop snippet if no content in page

Purpose:
========

When you start designing a page, you get a weird empty page
it was better in v8 with a default message prompting the user to drop a snippet

Specification:
==============

When a user starts designing a page he/she should get a default message prompting to drop a snippet (like we had it in v8).
[The drag&drop icon must be adapted: now snippets stand in the left side (and not in the upper part)]

addons/website/static/src/less/website.edit_mode.less

@@ -43,6 +43,11 @@
     }
 
     &.oe_structure.oe_empty, &[data-oe-type=html] {
+        &:empty {
+            height: 300px;
+            background: url(/website/static/src/img/drag_here.png) center no-repeat;
+            background-size: 380px;
+        }
         > p:empty:only-child {
             color: #aaa;
         }