[FIX] web: glitch with scroll and list editable

On a first edition of a view list editable, scrolling before the first
edition might cause an issue: the editing fields are higher than they
should.

This commit solves this.

This issue stems from jQuery .offset({value}) function which is broken
if the element we want to position:

- is in absolute postionning,
- is inside a scrolled element in non static positionning,
- has not both top and left css property setted,
- has no ancestor element between itself and the scrolled not in static
  positionning.

This issue happens less (and probably not at all) in saas-6 since the
last condition is most often not met thanks to this change:
 https://github.com/odoo/odoo/commit/1ccd87a#diff-27c072074221456684bfc5f150ca0bc9R876

This issue of jquery is solved since jquery 3.0.0-alpha1:
 https://github.com/jquery/jquery/commit/2d71594

( the issue is shown in https://jsfiddle.net/wpjrnggf/ and we can see it
is solved with jQuery 3.0.0-alpha1 https://jsfiddle.net/L6ykpjgy/ )

closes #8251
opw-647622

addons/web/static/src/js/view_list_editable.js

@@ -301,7 +301,7 @@
             var $cell = $(cell);
 
             field.set_dimensions($cell.outerHeight(), $cell.outerWidth());
-            field.$el.position({
+            field.$el.css({top: 0, left: 0}).position({
                 my: 'left top',
                 at: 'left top',
                 of: $cell