[Woo POS] Coupons: Handle coupons enabled/disabled in core

```
Coupons disabled (WooCommerce > Settings > General > Enable coupons):
Show a message and CTA to enable coupon functionality. Reload the list once the setting is enabled.
```

Ref: pdfdoF-6BG-p2#comment-7714

> This is handled along with all the other Woo settings. On Android, we fetch them on login
> Such settings are not expected to change often.