Refactoring: Add new Subscription class used to wrap all the WC

subscription related operations.

Changes:

* Clean up several modules moving subscription functionality to the
  new Subscription class.

* Modify all views and commands so the make use of the new
  Subscription class.

* Remove ExternalCredit model.

Signed-off-by: Birin Sanchez <birin.sanchez@puri.sm>
1 job for add_subscription_class in 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Deb
passed #160449
liberty
deb

00:00:32