App doesn't recognize any Metamask custom RPC (such as xDai)

@ryancreatescopy found a bug when trying to connecting a wallet while on the xDai network. The app doesn't recognize xDAI at all (or any custom RPC), so if your wallet is on xDAI (which due to current gas prices might be likely), we don't display any incorrect network message:

https://user-images.githubusercontent.com/8097623/109078432-2c095f00-76b2-11eb-89ed-4542e70d0104.mov

To reproduce, add the xDai network to Metamask:
https://www.xdaichain.com/for-users/wallets/metamask/metamask-setup

Solution may be to add xDai to our supported network list: https://github.com/ethereum/eth2.0-deposit/blob/dev/src/pages/ConnectWallet/web3Utils.ts#L17
Or a more sustainable/flexible solution may be to add some fallback logic that if the `chainId` isn't supported, we display a generic error that the network is not supported.