billing: Rename stripeToken to stripe_token.

When we started the billing system we started by following conventions used
in the Stripe documentation, but in hindsight it makes more sense to follow
conventions used in the Zulip codebase.

corporate/tests/test_stripe.py

@@ -226,7 +226,7 @@ class StripeTest(ZulipTestCase):
             if stripe_token is None:
                 stripe_token = stripe_create_token().id
             params.update({
-                'stripeToken': stripe_token,
+                'stripe_token': stripe_token,
                 'billing_modality': 'charge_automatically',
             })
         params.update(kwargs)

corporate/views.py

@@ -94,7 +94,7 @@ def initial_upgrade(request: HttpRequest) -> HttpResponse:
                         'lowball seat count',
                         "You must invoice for at least %d users." % (min_required_seat_count,))
                 seat_count = invoiced_seat_count
-            process_initial_upgrade(user, plan, seat_count, request.POST.get('stripeToken', None))
+            process_initial_upgrade(user, plan, seat_count, request.POST.get('stripe_token', None))
         except BillingError as e:
             error_message = e.message
             error_description = e.description