Added "newsletter_subscriptions" DB table
Added newsletter subscriptions data import from the previous Silex-based website YAML dump file
Added "Newsletter subscription" form

This form management has been quite hard to handle, because:
1) It doesn't live in its own HTML page, but included in any page
2) It has to be able to display itself in "error mode", included in its parent page
3) It has to be able to display a "success" Flash Message and hide itself, included in its parent page

* Added a "Blocks" Controller, with only one "newsletterSubscriptionFormBlock" Action for the moment
* Removed the @Cache annotation on our HomeController::indexAction, since it has to handle the newsletter notifications on the page
* Added a @Assert\Email annotation of our "email" field of the NewsletterSubscription Entity
* Added a new NewsletterSubscriptionType form type
* Added the management of "info" Flash Messages on the "alerts-display" Twig partial template
* Misc code style fixes
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
Better "newsletter subscription" partial form management

* No more ugly and security-critical Symfony Controller Action name inclusion in the newsletter form
* Replaced with a more secure, more elegant "_newsletter_subscription_form_master_controller" Session var
* Added a "newsletter-form.html.twig" partial to abstract the BlocksController rendering and its specific vars transmission
Merge pull request #1 from DrBenton/newsletter-subscription-management

Newsletter subscription management