Stripe: upgrade module and API, switch to Checkout (#12785)

* upgrade stripe module

* switch stripe api to latest version

* fix api version in tests

* start upgrading client and server

* client: switch to redirect

* implement checkout session creation for gems, start implementing webhooks

* stripe: start refactoring one time payments

* working gems and gift payments

* start adding support for subscriptions

* stripe: migrate subscriptions and fix cancelling sub

* allow upgrading group plans

* remove console.log statements

* group plans: upgrade from static page / create new one

* fix #11885, correct group plan modal title

* silence more stripe webhooks

* fix group plans redirects

* implement editing payment method

* start cleaning up code

* fix(stripe): update in-code docs, fix eslint issues

* subscriptions tests

* remove and skip old tests

* skip integration tests

* fix client build

* stripe webhooks: throw error if request fails

* subscriptions: correctly pass groupId

* remove console.log

* stripe: add unit tests for one time payments

* wip: stripe checkout tests

* stripe createCheckoutSession unit tests

* stripe createCheckoutSession unit tests

* stripe createCheckoutSession unit tests (editing card)

* fix existing webhooks tests

* add new webhooks tests

* add more webhooks tests

* fix lint

* stripe integration tests

* better error handling when retrieving customer from stripe

* client: remove unused strings and improve error handling

* payments: limit gift message length (server)

* payments: limit gift message length (client)

* fix redirects when payment is cancelled

* add back "subUpdateCard" string

* fix redirects when editing a sub card, use proper names for products, check subs when gifting

website/client/src/components/settings/subscriptionOptions.vue

@@ -27,7 +27,10 @@
     </b-form-group>
     <payments-buttons
       :disabled="!subscription.key"
-      :stripe-fn="() => showStripe({subscription:subscription.key, coupon:subscription.coupon})"
+      :stripe-fn="() => redirectToStripe({
+        subscription: subscription.key,
+        coupon: subscription.coupon,
+      })"
       :paypal-fn="() => openPaypal({url: paypalPurchaseLink, type: 'subscription'})"
       :amazon-data="{
         type: 'subscription',