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>

apps/web/src/app/(dashboard)/settings/billing/billing-plans.tsx

@@ -4,7 +4,7 @@ import { useState } from 'react';
 
 import { AnimatePresence, motion } from 'framer-motion';
 
-import { PriceIntervals } from '@documenso/ee/server-only/stripe/get-prices-by-interval';
+import type { PriceIntervals } from '@documenso/ee/server-only/stripe/get-prices-by-interval';
 import { useIsMounted } from '@documenso/lib/client-only/hooks/use-is-mounted';
 import { toHumanPrice } from '@documenso/lib/universal/stripe/to-human-price';
 import { Button } from '@documenso/ui/primitives/button';