Add endpoint for subscription webhook

[AdamVig]

• [x] Add /subscription/hook that receives webhooks when subscription is updated or payment is received.
• [x] Add Subscription entity that stores status and Stripe customer ID, relates to Organization
• [x] Add Subscription middleware to check subscription status on every PUT, POST, and DELETE request (returns 402 Payment Required when invalid)
• [x] Add GET /subscription for checking subscription status

[AdamVig]

The new Subscription entity contains the following fields:

• subscriptionID
• organizationID
• stripeCustomer: unique ID for accessing Stripe customer information
• valid: 0 for invalid (when status is EXPIRED or TRIAL_EXPIRED), 1 for valid (when status is TRIAL or VALID). Quick way of discerning whether or not user should be allowed to use the app.
• subscriptionStatusID: relates to the new Subscription Status entity, described below
• statusUntil: datetime that represents when the current subscription status ends. Relevant for TRIAL and VALID statuses. Should be null when the status is EXPIRED or TRIAL_EXPIRED.

The new Subscription Status entity is simply a name and subscriptionStatusID. It allows the apps to use an enum for customizing behavior based on the current subscription status. The current statuses are (the list numbers represent the ID of each status):

1. TRIAL: default state.
2. TRIAL_EXPIRED: trial has ended, user has not paid for a subscription.
3. VALID: user has paid for a subscription.
4. EXPIRED: user's payment is overdue.
5. CANCELED: user has cancelled subscription.