Refactor PerunToken to generic ERC20Token #16
Description
Currently, we always use the PerunToken contract which extends the ERC20 contract when we want to use ERC20 tokens as the asset.
However, this may lead to the confusion that the perun-eth-backend only supports the custom PerunToken as an ERC20 asset instead of general ERC20 tokens according to the OpenZeppelin standard.