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/create-customer.ts

@@ -1,31 +0,0 @@
-import { stripe } from '@documenso/lib/server-only/stripe';
-import { getSubscriptionByUserId } from '@documenso/lib/server-only/subscription/get-subscription-by-user-id';
-import { prisma } from '@documenso/prisma';
-import { User } from '@documenso/prisma/client';
-
-export type CreateCustomerOptions = {
-  user: User;
-};
-
-export const createCustomer = async ({ user }: CreateCustomerOptions) => {
-  const existingSubscription = await getSubscriptionByUserId({ userId: user.id });
-
-  if (existingSubscription) {
-    throw new Error('User already has a subscription');
-  }
-
-  const customer = await stripe.customers.create({
-    name: user.name ?? undefined,
-    email: user.email,
-    metadata: {
-      userId: user.id,
-    },
-  });
-
-  return await prisma.subscription.create({
-    data: {
-      userId: user.id,
-      customerId: customer.id,
-    },
-  });
-};