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/prisma/schema.prisma

@@ -21,6 +21,7 @@ enum Role {
 model User {
   id                   Int                  @id @default(autoincrement())
   name                 String?
+  customerId           String?              @unique
   email                String               @unique
   emailVerified        DateTime?
   password             String?
@@ -34,7 +35,7 @@ model User {
   accounts             Account[]
   sessions             Session[]
   Document             Document[]
-  Subscription         Subscription?
+  Subscription         Subscription[]
   PasswordResetToken   PasswordResetToken[]
   twoFactorSecret      String?
   twoFactorEnabled     Boolean              @default(false)
@@ -72,18 +73,16 @@ enum SubscriptionStatus {
 model Subscription {
   id                Int                @id @default(autoincrement())
   status            SubscriptionStatus @default(INACTIVE)
-  planId            String?
-  priceId           String?
-  customerId        String
+  planId            String             @unique
+  priceId           String
   periodEnd         DateTime?
-  userId            Int                @unique
+  userId            Int
   createdAt         DateTime           @default(now())
   updatedAt         DateTime           @updatedAt
   cancelAtPeriodEnd Boolean            @default(false)
 
   User User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
-  @@unique([customerId])
   @@index([userId])
 }