Refactor PerunToken to generic ERC20Token

[cryptphil]

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.