Move base64 migration prompt to corner and only when the pad is stored

customize.dist/src/less2/include/corner.less

@@ -81,6 +81,10 @@
                 line-height: 200px;
             }
         }
+        &.cp-corner-big {
+            width: 400px;
+            height: 250px;
+        }
 
         .cp-corner-actions {
             min-height: 30px;