Improve message signature request

[alfetopito]

Summary

Message signing currently displays no useful info to the user, leading to a potential exploitation avenue.

For example, when signing something, this is what I see:

And this is what I would expect to see, take from Metamask extension:

Other wallets implement the data visualization in different ways, but IMO Metamask is the one that shows the info in the clearest manner.

So, this is a request on behalf of users to show the data being signed

Additional info

I came across this when testing our app (CowSwap), which heavily relies on offline signatures. In this PR, I'm making the app compatible with MEW mobile wallet https://github.com/gnosis/gp-swap-ui/pull/704 You can use this app to try it out for yourselves https://pr704--gpswapui.review.gnosisdev.com/ The app works on Mainnet, Rinekby and xDai, so feel free to try without spending any real funds.