[IMP] web_kanban: optimize drag and drop

This is a really difficult optimization.  Based on various benchmarks
and some hard thinking, we discovered that this change will shave
approximately 100ms for each drag and drop performed by a normal user.

We believe that this is a safe optimization in most cases (but we didn't
test it on some architectures, such as ARM or Solaris).  It is also
compatible with most SQL JSON-enabled databases.

addons/web_kanban/static/src/js/kanban_column.js

@@ -73,7 +73,7 @@ var KanbanColumn = Widget.extend({
         this.$el.sortable({
             connectWith: '.o_kanban_group',
             revert: 150,
-            delay: 100,
+            delay: 0,
             items: '> .o_kanban_record',
             helper: 'clone',
             cursor: 'move',