[FEATURE] Code Restricted Ticket Purchases

[wendylbolm]

Is your feature request related to a problem? Please describe.
A nonprofit wants to sell tickets that are limited so that only people who have a special code can purchase them.

Describe the solution you'd like
Nonprofit sets up a code for ticket access (like a coupon code)
Nonprofit sets up a ticket level that only allows for purchase if the code is entered
Donors who go to the ticket page can see the code-only ticket level but cannot purchase it unless they have a code

Additional context
This is the first time this has been requested; it may be a good feature for higher-end gala dinners with exclusive ticket levels.