Add Token Search for other networks

[PeterTheOne]

On the Add Tokens screen there is a search for tokens, the data comes from https://github.com/MetaMask/eth-contract-metadata and only works for the Ethereum network. Token contracts (most probably) have different addresses or don't exist on different networks, in addition there are additional tokens on other networks.

It would be nice to have a working search for different networks. On solution would be that https://github.com/MetaMask/eth-contract-metadata has lists for different networks that could be used in metamask.
Another solution would be to disable the search for other networks than Ethereum.

Related issue: #4518