Support for the channelUrl parameter of FB.init().

Reference : https://developers.facebook.com/docs/reference/javascript/

Conflicts:
	DependencyInjection/FOSFacebookExtension.php
	Tests/Templating/Helper/FacebookHelperTest.php
Migrated the fos_facebook.channel.url parameter to a route, removed an unused parameter.

Conflicts:
	DependencyInjection/FOSFacebookExtension.php
	Tests/Templating/Helper/FacebookHelperTest.php
Changed the RouterInterface to a UrlGeneratorInterface. Formatting changes.

Conflicts:
	Tests/Templating/Helper/FacebookHelperTest.php
Changed implementation to interface in test.

Conflicts:
	Tests/Templating/Helper/FacebookHelperTest.php
Changed implementation to interface (bis).

Conflicts:
	Tests/Templating/Helper/FacebookHelperTest.php
Adding a reference of how to include the route for support the channel.html