Moves `update_billing_method_of_current_plan` to the BillingSession
abstract class.
Adds a helper function for support views for the realm case:
`update_realm_billing_method`.
Moves `update_sponsorship_status` to BillingSession abstract class
as `update_customer_sponsorship_status`.
Updates the support views to have a helper for updating this on a
realm: `update_realm_sponsorship_status`.
Makes `approve_sponshorship` an abstract method in BillingSession
abstract base class and moves the implementation for realms to the
RealmBillingSession child class.
Adds `approve_realm_sponsorship` helper function that's used in
the support view and initiates the billing session.
This moves the logic for `attach_realm_discount`, which is used in
the support view, to be in the BillingSession class.
Updates the function name to be `attach_discount_to_customer` so
that the context is generalized vs realm specific.
Updates RealmBillingSession implementation to account for actions
that are initiated by a support admin user.
Also moves the helper function `get_discount_for_realm` that is
only used in support views to `corporate/lib/support.py`.
Lauryn Menard
Eeshan Garg