Implementation Hook provider for newsletter subscribing. Added configurable possibility to hook to main user registration workflow

With this PR Newsletter will become hook provider, and subscribing to the newsletter can be attached to places, which are form_edit hook subscribers. Most important one is main user registration procedure. It is tested with this procedure,

Note:
- For existing installations with module version 2.2.1 (last development), have to manually change module version to 2.2.0. This is just to force upgrade procedure to install provider bundles.