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>

.env.example

@@ -77,14 +77,10 @@ NEXT_PRIVATE_MAILCHANNELS_DKIM_PRIVATE_KEY=
 NEXT_PRIVATE_STRIPE_API_KEY=
 NEXT_PRIVATE_STRIPE_WEBHOOK_SECRET=
 NEXT_PUBLIC_STRIPE_COMMUNITY_PLAN_MONTHLY_PRICE_ID=
-NEXT_PUBLIC_STRIPE_COMMUNITY_PLAN_YEARLY_PRICE_ID=
-NEXT_PUBLIC_STRIPE_FREE_PLAN_ID=
 
 # [[FEATURES]]
 # OPTIONAL: Leave blank to disable PostHog and feature flags.
 NEXT_PUBLIC_POSTHOG_KEY=""
-# OPTIONAL: Defines the host to use for PostHog.
-NEXT_PUBLIC_POSTHOG_HOST="https://eu.posthog.com"
 # OPTIONAL: Leave blank to disable billing.
 NEXT_PUBLIC_FEATURE_BILLING_ENABLED=