[WIP] PIM-3207: Apply the user channel in the edit product page

TODO:
 - [ ] another bug has been raised by this fix. When you add a scopable attribute to a product. The icon which allows to remove it is only available on the first channel. So the scenario "Successfully remove a scopable attribute from a product" is failing right now.
 - [ ] check that it works with mass edit and a  Behat for this

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y except "Successfully remove a scopable attribute from a product"
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | N
| DB schema updated?   | N
| Migration script?    | -
| Doc PR               | -
Or you could just remove the last `return null;` :tada: