Export payout_tx so that it can be consumed when calculating whether rewards are pending payout.

Closes minds#2273 (closed)

Depends on front!1465 (merged)

Useful Queries:

// select users rewards from table.
SELECT * FROM minds.token_rewards WHERE user_guid = 1228323316385714193;

// valid reward that is already paid out - change user_guid and date
INSERT INTO minds.token_rewards (user_guid, reward_type, date, multiplier, payout_tx, score, token_amount, tokenomics_version) VALUES (1228323316385714193, 'liquidity', '2021-04-25', 2, '1', 2, 2, 2);

// not yet paid out - change user_guid and date
INSERT INTO minds.token_rewards (user_guid, reward_type, date, multiplier, payout_tx, score, token_amount, tokenomics_version) VALUES (1228323316385714193, 'liquidity', '2021-04-25', 2, null, null, null, 2);

// clear tables
TRUNCATE minds.token_rewards;