Feature/bounty contract integration

[dohaki]

Still WIP-ish... You can fund issues but remaining TODOs are:

• Open modal to specify amount, token, etc when funding
• Refunding contributions
• Whole fulfillment flow

To test run:

1. yarn hardhat:node
2. yarn contracts:deploy
3. Set the contract address in .env.local for NEXT_PUBLIC_BOUNTIES_CONTRACT_ADDRESS

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