Hide the product / order forms if there are no stores

We can place the trait in \Drupal\commerce_store\Form\StoreWarningTrait.

The order and product form would use the trait, call $this->buildStoreWarning(), and if the output is not empty, replace the form with the warning.
The buildStoreWarning() method would do an entity query to get the number of stores, if the count is 0, return the warning element.

I've committed an improved version of this in https://github.com/drupalcommerce/commerce/commit/e837ba054b07d5a1857924...

I ended up removing the trait and repeating the code in both forms, since it's a small amount of code reliant on external dependencies, and because I wanted different messages on the two forms.

Worth mentioning:

 +      $options = [
 +        'query' => [
 +          'destination' => Url::fromRoute('<current>')->toString(),
 +        ],
 +      ];
 +      $link = Link::createFromRoute('Add a new store.', 'entity.commerce_store.add_page', [], $options);

Turns out the current url can be received using the trick above. Didn't know that.
Also decided to hardcode the route to the add page, since it follows standards and can't change.