Prompt for missing chain (EIP-3085)

[edmundedgar]

If the user doesn't have the network specified we should be able to prompt for it:

https://eips.ethereum.org/EIPS/eip-3085
https://docs.metamask.io/guide/rpc-api.html#other-rpc-methods

[edmundedgar]

Done in c4585cc

Currently editing the chains.json file from chains.network to add the XDAI logo and block explorer urls, and also tell it not to prompt to switch to mainnet (which metamask refuses) or rinkeby (which only has infura rpc urls, and we don't want everyone installing our infura key in their browsers).

Hopefully we can get a long-term solution to the first issues depending on:
ethereum-lists/chains#202
ethereum-lists/chains#59