Feature/stripe_integration

- [ ] Ready to merge
- [x] Finalise user to pro path
  - Navigate to /signup
  - Click pay now
  - Enter details
  - Stripe charges first month
     - If fail, redirect homepage and flash error,
     - If success
       - Update db
       - Update session and redirect to homepage
       - Send email.
  - If payment is missed repeatedly, remove proStatus
    - Update db
    - Update session // #1012 required
    - Send email
- [x] Respond to all web hooks with 200
- [x] Create customer model
- [x] Handle cancel subscriptions / non payments
- [x] Handle declined cards
- [x] Handle user not signed in
- [x] Deal with previous customer re subscribing
- [x] Add database methods
- [ ] Write tests for additional code