Add Dapp detection

[bbondy]

We should give a notification prompt to ask the user to install MetaMask if they visit a Dapp like cryptokitties.

See:
https://github.com/brave/browser-laptop/blob/master/app/browser/reducers/dappReducer.js

Test plan

• With a new profile, go to cryptokitties.co and you should see a prompt to install from the website.
• After it is installed, you should not get this prompt anymore.
• You should not see this prompt on a new profile on other sites.

[lukemulks]

Would it be possible for us to preload MetaMask, as we do with b-l?

We provided a security and blockchain advantage in the market when we shipped with MM pre-installed (and inactive).

Recalling when the extension store recently removed the legit MM along with some duplicates - Brave had the advantage and benefit of being able to confirm to our user base (and the public) that our pre-loaded MM extension was authentic and up to date.

Also allows for Brave to state that we ship a web3-ready browser.

There may be some reason I'm unaware of why it's better not to do this, but mentioning as I found myself looking for MM in the dev channel.

[bbondy]

We never pre-installed metamask, but we just made it really easy to install via the Dapp detection and by having an option directly in settings to download and install it in 1 step.

[bsclifton]

When looking at this, please see this issue regarding the integration done with the Muon version of Brave:
brave/browser-laptop#15221