Add support for ERC-20 tokens (specifically DAI)

[ErikBjare]

Enabling people to donate with whatever token they'd like (especially stablecoins) would be a huge step towards making Thankful less financially complicated for users (fewer risks holding DAI than ETH).

Also, aligning our interfaces with the ERC-20 standard and abandoning legacy-transfers of ETH in favor of WETH would be nice.

I, for one, would definitely prefer supporting creators by sending them DAI rather than ETH.