Change endpoint for discount code and access code.

[prateekj117]

Frontend issue for fossasia/open-event-server#6027

Describe the bug
Change endpoint for retrieving discount code using event_id as in fossasia/open-event-server#6208.

Expected behaviour
Discount code and access code logic should work properly.

[prateekj117]

@mariobehling @uds5501 Please add the tag of high priority and add it to this week milestone.

[kushthedude]

Please refer to Gitter Conversation.

[prateekj117]

@CosmicCoder96 The access-code changes were not done in the previous PR. When entering the Promotional Code, it checks for both the access-codes and discount-codes. Working on the access-code part.