Support ERC-20 tokens

[guidanoli]

We could either:

• Not support ETH anymore and just support ERC-20 tokens
• Support ETH and ERC-20 tokens

The difficulty part there is encoding value in the application state. We could add an optional token field to the AppBounty struct. If not provided, it's assumed the bounty only accepts sponsorships in ETH. Otherwise, it only accepts sponsorships in the provided ERC-20 token. The type of currency would be determined by whoever creates the bounty.

In the back-end, we'd have to:

• Store the token field of "Create app bounty" inputs
• Process "Add sponsorship" inputs from the ERC-20 token portal
• Check if sponsorships use the currency specified in the bounty
• Handle "Withdraw sponsorship" requests dealing with ERC-20 tokens

In the web front-end, we'd have to tweak:

• "Create bounty" page (add sponsorship currency option)
• "Bounty info" page (adjust total prize display)
• "Sponsor bounty" page (adjust sponsorship value display)
• Profile cards of sponsors (adjust total sponsorship value display)

It would also be good to test these new cases in Lua.

[guidanoli]

For optional fields, we can use the omitempty JSON option. Source: https://pkg.go.dev/encoding/json

[guidanoli]

I'll go with the easiest route, which is to accept only ERC-20 tokens, and not support native ETH anymore. ETH can be wrapped in ERC-20 tokens and 1-to-1 equivalence can be enforced by contract logic.