corporate: Create change plan tier function for BillingSession.

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.

analytics/views/support.py

@@ -56,13 +56,13 @@ if settings.BILLING_ENABLED:
     from corporate.lib.stripe import (
         RealmBillingSession,
         get_latest_seat_count,
-        switch_realm_from_standard_to_plus_plan,
         void_all_open_invoices,
     )
     from corporate.lib.support import (
         approve_realm_sponsorship,
         attach_discount_to_realm,
         get_discount_for_realm,
+        switch_realm_from_standard_to_plus_plan,
         update_realm_billing_method,
         update_realm_sponsorship_status,
     )