Moves the 'update_license_ledger_for_manual_plan' function
to the 'BillingSession' abstract class.
This refactoring will help in minimizing duplicate code while
supporting both realm and remote_server customers.
Also adds `SWITCH_PLAN_TIER_AT_PLAN_END` for `CustomerPlan`
which will be used to mark status of remote server legacy
plans which are scheduled for an upgrade.
The logic for BillingSession.is_sponsored_or_pending would be the
same for all three child classes of BillingSession, so this should
be a method on the BillingSession abstract class.
Creates a process_support_view_request method for BillingSession
to process the various support requests that relate to the billing
system.
Moves approve_realm_sponsorship, update_realm_sponsorship_status,
and attach_discount_to_realm to this new BillingSession method.
Adds a new abstract property to BillingSession to have a string
value, billing_entity_display_name, to use for support messages
sent when these requests are processed.
The "send_invoice" and "charge_automatically" strings used by stripe
for the `collection_method` are referred to both as the "billing
method" and "billing modality" in the billing code.
Because we send this as data to stripe as either `collection_method`
or `billing_modality`, renames any references that are any form of
"billing method".
Updates do_change_plan_to_new_tier in BillingSession to use an
enum for the value returned when checking for a valid change
between two plan tier types. This makes it more explicit that
the implementation for a valid upgrade in plan tier will be
different from a valid downgrade in plan tier.
This commit refactors 'sponsorship_request' view and adds
'BillingSession.get_sponsorship_request' method.
This refactoring will help in minimizing duplicate code
while supporting both realm and remote_Server customers.
This commit refactors 'event_status' view and adds
'BillingSession.get_event_status' method.
This refactoring will help in minimizing duplicate code
while supporting both realm and remote_server customers.
Moves and generalizes `switch_realm_from_standard_to_plus_plan`
in stripe.py to be a more general function for changing a
CustomerPlan to a new and valid tier, `do_change_plan_to_new_tier`.
Adds a helper function with the previous function name to be used
for the support view and management command for changing a realm
from the Standard plan tier to the Plus plan tier.
Renames CustomerPlan.SWITCH_NOW_FROM_STANDARD_TO_PLUS to be more
generic, CustomerPlan.SWITCH_PLAN_TIER_NOW.
Because the plan tier change is immediate, moves the code to end
the current plan with the old tier and create the new plan with
the new tier from `make_end_of_cycle_updates_if_needed` to instead
be in a separate helper function, `switch_plan_tier`.
