Miscellaneous backend changes

• Implemented a mechanism to disable user notification emails for a specific product (upon request by the product owner).
• Fixed “Special Coupons” handling in the new “Setup Checklist” system.
• Added exchange rates in the API endpoint related to the Developer Dashboard to make the FX currency work.
• Fixed an edge case issue that was happening when deleting a user who was already associated with another user with the same email address.
• Fixed an issue in the store creation endpoint to auto-assign default values.
• Various visual improvements to our blog.