[FIX] web: caret misplaced

Due to the library algorithm to place the datetimepicker,
the div containing the input was no longer relative so the caret was misplaced.
Put back the position relative and place the datetimepicker relative to the input.

addons/web/static/src/js/widgets/date_picker.js

@@ -46,8 +46,8 @@ var DateWidget = Widget.extend({
         this.$input.focus(function(e) {
             e.stopImmediatePropagation();
         });
-        this.$el.datetimepicker(this.options);
-        this.picker = this.$el.data('DateTimePicker');
+        this.$input.datetimepicker(this.options);
+        this.picker = this.$input.data('DateTimePicker');
         this.$input.click(this.picker.toggle.bind(this.picker));
         this.set_readonly(false);
         this.set_value(false);

addons/web/static/src/less/datepicker.less

@@ -13,6 +13,8 @@
 }
 
 .o_datepicker {
+    position: relative;
+
     .o_datepicker_input {
         width: 100%;
     }