Change running subscriptions

AppSubscription::change() changes price, unit, name, description or
discount of a running subscription, prorated right away (APPLY_NOW) or
from the next period on (APPLY_PERIOD_END, shown as scheduled_change);
cancelScheduledChange() drops a scheduled one. Adds the
subscription.updated event type and documents proration_not_possible
and credit_exceeds_bill.

tests/Id/CheckoutSessionsTest.php

@@ -187,5 +187,6 @@ class CheckoutSessionsTest extends TestCase
         $this->assertContains('subscription.activated', EventType::all());
         $this->assertContains('subscription.trial_will_end', EventType::all());
         $this->assertContains('subscription.discount_ended', EventType::all());
+        $this->assertContains('subscription.updated', EventType::all());
     }
 }