Fix/discounts UI

[ricardobrg]

Implements the Polaris discount UIs for TIKI app.

This PR implements the UIs and the backend functions to save the UI data.

The save/edit functions and the dynamic fields will be handled by tiki/integrations#103, tiki/integrations#105, tiki/integrations#106 and tiki/integrations#107, with specific backend functions.

The helper messages and dashboard UI will be added in tiki/tiki-shopify#5

How to test

in frontend folder run npm run start. The following routes are implemented:

• /ui - Dashboard placeholder. Will be the root of the app UI in Shopify admin backend.
• /ui/discounts/product - Product discounts UI
• /ui/discounts/order - Order discounts UI

The UIs are equal but we need both for configuring discounts that will be applied to just a product or the whole order, following Shopify default UX.

[ricardobrg]

Merged the HMAC code

[ricardobrg]

Added the code for handling the data saving in the backend.