Investigate & add support for Pretix Discounts

[marcoacierno]

https://docs.pretix.eu/en/latest/api/resources/discounts.html

Use case: https://docs.pretix.eu/en/latest/user/events/guides/groups.html

[marcoacierno]

this could be useful: https://docs.pretix.eu/en/latest/development/algorithms/pricing.html#discounts

[marcoacierno]

the best and easiest solution is probably to see if we use our extended API plugin to expose the discount engine via API. So the FE could check for discounts. The only side effect is that it requires a request so there will be a tiny delay when going to the checkot page