Add redeem functionality

Merging this PR should put us in good shape to complete the database functionality and corresponding UI functionality. Changes in this PR:

Fix bugs in "buy ptoken" flow
- Fix handling of decimal amounts
- Fix call to purchase method of contract where the incorrect value was input
Description data now included with POST call via request_redemption() function
Adds a function called completePtokenRedemption which calls into the contract to complete the redemption

raid-guild / gitcoinco-web