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/get-prices-by-interval.ts

@@ -1,4 +1,4 @@
-import Stripe from 'stripe';
+import type Stripe from 'stripe';
 
 import { stripe } from '@documenso/lib/server-only/stripe';
 
@@ -7,7 +7,14 @@ type PriceWithProduct = Stripe.Price & { product: Stripe.Product };
 
 export type PriceIntervals = Record<Stripe.Price.Recurring.Interval, PriceWithProduct[]>;
 
-export const getPricesByInterval = async () => {
+export type GetPricesByIntervalOptions = {
+  /**
+   * Filter products by their meta 'type' attribute.
+   */
+  type?: 'individual';
+};
+
+export const getPricesByInterval = async ({ type }: GetPricesByIntervalOptions = {}) => {
   let { data: prices } = await stripe.prices.search({
     query: `active:'true' type:'recurring'`,
     expand: ['data.product'],
@@ -19,8 +26,10 @@ export const getPricesByInterval = async () => {
     // eslint-disable-next-line @typescript-eslint/consistent-type-assertions
     const product = price.product as Stripe.Product;
 
+    const filter = !type || product.metadata?.type === type;
+
     // Filter out prices for products that are not active.
-    return product.active;
+    return product.active && filter;
   });
 
   const intervals: PriceIntervals = {