Pool shares token

Pool shares token template used by btn.group built on the Secret network blockchain.

Changes made to standard SNIP-20/SNIP-21 implementations

1. Remove ability to change initial minters.
2. Remove all reference to admin.
3. Remove ability to change contract status.
4. Remove ability to deposit and redeem native tokens.
5. Remove ability to set initial balances.

References

1. https://github.com/enigmampc/snip20-reference-impl
2. https://github.com/SecretFoundation/SNIPs/blob/master/SNIP-20.md
3. https://github.com/enigmampc/scrt-finance-rewards/tree/master/contracts/gov-token
4. https://github.com/enigmampc/SecretSwap/blob/master/contracts/secretswap_token/README.md
5. https://github.com/btn-group/buttcoin