Stop using credits but only Mainnet NUM in the Capture system

Reasons why we needed Capture credits:

1. Cost efficient (no gas required)
2. Cannot be liquidated => Can use it as on-boarding gifts to encourage users using Capture App
3. App Store does not allow users to purchase crypto, can only buy credits/points

However, after the NUM migration

1. Mainnet NUM is cost efficient, plus, we are the only validator at the moment (i.e. can recycle the gas)
2. Hard to liquidate, can almost say it is not possible to liquidate for small amount (because users need to bridge NUM coins to ETH NUM first (very high costs)
3. Maybe we should see App Store purchase as one of the ways to purchase NUM coin, i.e. once the purchase is done, we can send NUM coin to users' wallets.

Pros of stop using credits: much simpler to maintain in the tech side and more clear to users and to the finance team.
Cons of stop using credits: still have some risks that users may try to hack and get NUM coins. However, I believe we should be able to manage this risk.

┆Issue is synchronized with this Asana task by Unito
┆Created By: Tammy Yang