Add a dropdown in Admin UI to choose which discount should be used

[ricardobrg]

As a developer I need to show a dropdown with the list of active discounts to the user so that the user can choose which discount to use in the offer prompt

What it is now

There is no UI for that.

What it should be

A simple UI with a dropdown button and a save button.

Technical details

The options should come from backend or graphql as described in #52. The save button should call the endpoint to save the discount information.

Acceptance criteria

1. In the admin UI, an endpoint can be called that lists all TIKI app discounts

[ricardobrg]

Dropdown UI done with placeholder discounts.