PIM-2574: No popup saying there is unsaved changes

When a product form is submitted with invalid values, the page in
reloaded with theses values (even the good one) and if we click on back
to the grid there is no warning message we are leaving a form with
unsaved values

| Q                    | A
| -------------------- | ---
| Bug fix?             | Yes
| New feature?         | No
| BC breaks?           | No
| CI currently passes? | ?
| Tests pass?          | Yes
| Scenarios pass?      | ?
| Checkstyle issues?*  | No
| PMD issues?**        | No
| Changelog updated?   | Yes
| Fixed tickets        | PIM-2574
| Doc PR               | No
src/Pim/Bundle/UIBundle/Resources/views/Form/pim-fields.html.twig