Account Module: block or replace injected web3 provider if none is installed

[sohkai]

On a browser without an injected web3 provider, the "injected" column currently displays "Unknown":

We should either remove this provider and add a different one (e.g. WalletConnect), or block it and say "no injected wallet available". I would prefer the former but I imagine this may be more difficult?

[sohkai]

We will track account module improvements in the client repo: aragon/client#1364