Reverted the @Cache annotation on our HomeController::indexAction

The page content modifications (Flash Messages, newsletter form state) only occur on POST requests, which are not handled by the cache, so... let's enable the cache again :-)
Added the "email" field uniqueness constraint management

* Added a "@UniqueEntity" Validator on our NewsletterSubscription Entity
* Because we don't want anyone to guess which email addresses are registered to our newsletter, we add a manual uniqueness check in the BlocksController::newsletterSubscriptionFormBlockAction and display a "successful subscription" message when the email address is already registered