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/migrations/20231206073509_add_multple_subscriptions/migration.sql

@@ -0,0 +1,31 @@
+/*
+  Warnings:
+
+  - A unique constraint covering the columns `[planId]` on the table `Subscription` will be added. If there are existing duplicate values, this will fail.
+  - A unique constraint covering the columns `[customerId]` on the table `User` will be added. If there are existing duplicate values, this will fail.
+  - Made the column `planId` on table `Subscription` required. This step will fail if there are existing NULL values in that column.
+  - Made the column `priceId` on table `Subscription` required. This step will fail if there are existing NULL values in that column.
+
+*/
+-- Custom migration statement
+DELETE FROM "Subscription" WHERE "planId" IS NULL OR "priceId" IS NULL;
+
+-- DropIndex
+DROP INDEX "Subscription_customerId_key";
+
+-- DropIndex
+DROP INDEX "Subscription_userId_key";
+
+-- AlterTable
+ALTER TABLE "Subscription" ALTER COLUMN "planId" SET NOT NULL,
+ALTER COLUMN "priceId" SET NOT NULL;
+
+-- AlterTable
+ALTER TABLE "User" ADD COLUMN     "customerId" TEXT;
+ALTER TABLE "Subscription" DROP COLUMN "customerId";
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Subscription_planId_key" ON "Subscription"("planId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "User_customerId_key" ON "User"("customerId");