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

config.json.example

@@ -71,6 +71,7 @@
   "SLACK_URL": "https://hooks.slack.com/services/some-url",
   "STRIPE_API_KEY": "aaaabbbbccccddddeeeeffff00001111",
   "STRIPE_PUB_KEY": "22223333444455556666777788889999",
+  "STRIPE_WEBHOOKS_ENDPOINT_SECRET": "111111",
   "TRANSIFEX_SLACK_CHANNEL": "transifex",
   "WEB_CONCURRENCY": 1,
   "SKIP_SSL_CHECK_KEY": "key",