Changes to pricing page

Includes:

- [x] Make price numbers bigger
- [x] Remove "Explorer", "Corporate" and "Journalist" plans
- [x] Remove storage and CPU limits
- [x] Add "Enterprise" plan for $199 with unlimited datasets
- [x] Alter "Data Scientist" plan to allow 100 datasets
- [x] Tick newsletter checkbox on signup page by default
- [x] A separate `/thankyou` page, rather than a flash on the homepage