Admin Sales Page: Make Ticket number and Sales Number sortable through backened

[mariobehling]

Tickets and Sales numbers on the admin sales page are computed properties and it is not easily possible to sort them on the frontend itself. Even if we could sort them here it could slow down the page due to potentially large lists.

The sorting is very important to the admin, because it enables the admin to identify top organizers.

Please implement a sorting / list order according to ticket sales and revenue through the backend.