Add support for easy flashes handling

[sstok]

It's a common practice to show a flash message after a redirect, rather than having to get a session manually (ensuring its started) you can use now use the withFlash() method.

Example:

return RouteRedirectResponse::toRoute('park_manager.client_section.webhosting.account.ftp.user_list')
                ->withFlash('success', 'translation.id', 'arguments');

When the arguments are empty (null) the message is past added to FlashBag as a string, otherwise it's an array like ['message' => 'message.id' => ['id' => 200]].

Translations and special formatting must still be done in the template.