feat: add multi subscription support (#734)

## Description

Previously we assumed that there can only be 1 subscription per user.
However, that will soon no longer the case with the introduction of the
Teams subscription.

This PR will apply the required migrations to support multiple
subscriptions.

## Changes Made

- Updated the Prisma schema to allow for multiple `Subscriptions` per
`User`
- Added a Stripe `customerId` field to the `User` model
- Updated relevant billing sections to support multiple subscriptions

## Testing Performed

- Tested running the Prisma migration on a demo database created on the
main branch

Will require a lot of additional testing.

## Checklist

- [ ] I have tested these changes locally and they work as expected.
- [ ] I have added/updated tests that prove the effectiveness of these
changes.
- [X] I have followed the project's coding style guidelines.

## Additional Notes

Added the following custom SQL statement to the migration:

> DELETE FROM "Subscription" WHERE "planId" IS NULL OR "priceId" IS
NULL;

Prior to deployment this will require changes to Stripe products:
- Adding `type` meta attribute

---------

Co-authored-by: Lucas Smith <me@lucasjamessmith.me>

packages/ee/server-only/stripe/webhook/on-subscription-deleted.ts

@@ -1,4 +1,4 @@
-import { Stripe } from '@documenso/lib/server-only/stripe';
+import type { Stripe } from '@documenso/lib/server-only/stripe';
 import { prisma } from '@documenso/prisma';
 import { SubscriptionStatus } from '@documenso/prisma/client';
 
@@ -7,12 +7,9 @@ export type OnSubscriptionDeletedOptions = {
 };
 
 export const onSubscriptionDeleted = async ({ subscription }: OnSubscriptionDeletedOptions) => {
-  const customerId =
-    typeof subscription.customer === 'string' ? subscription.customer : subscription.customer?.id;
-
   await prisma.subscription.update({
     where: {
-      customerId,
+      planId: subscription.id,
     },
     data: {
       status: SubscriptionStatus.INACTIVE,