[FIX] web: fix styling in many2many tag input

Since Chrome 53.0.2785, the input select is hover the existing tag.
With this fix, the existing tag are displayed at the right side of
the input to create a new one.

This commit closes #13655

Courtesy of @8cells

addons/web/static/src/less/enterprise_compatibility.less

@@ -300,7 +300,7 @@ div.o_checkbox {
 
         > .oe_form_field {
             .o-flex(1, 0, 100px);
-
+            position: initial;
             input {
                 border: none;
             }