Merge phpBB request and Symfony Request

Both phpBB and Symfony provides and uses a request object.
While they don't have exactly the same features their goal is the same: giving a safe access and an easier/unified interface to the super globals content.

I think we should merge them by making our request object extending Symfony's one (it's needed because we can't alter Symfony's code but we have full control over our own).

To remain BC, the new request class will have to include all the existing public methods and fields of the current \phpbb\request\request class and the current \phpbb\symfony_request class will need to be marked as deprecated and extends the new request class.