Allow manual enable for legacy sites.

[MicahZoltu]

What problem are you trying to solve?
There are a number of older sites that do not call .enable(), they just try to access web3.currentProvider. Without taking MetaMask out of privacy mode, I would like a way to be able to approve the site so it gets an injected web3.currentProvider so I can continue using the dapp.

Describe the solution you'd like
Put a button in the extension UI for "Allow this site to see your MetaMask address" (wording could probably be improved).

Additional context
Part of the idea with dapps is that they can be deployed to things like IPFS and live forever, even after the author is gone. Also, if you audit a dapp you may not want to spend (or have the) time to re-audit it so you choose to not update to a newer version of the dapp that supports .enable(). In these cases, there is value in allowing MetaMask to be backward compatible for as long as possible.

Toggling off Privacy Mode is not what I want because that disables privacy mode for every open tab I have. I want to just allow a specific site access through the UI.

[bdresser]

This is also a relevant feature for when we enable privacy mode by default

[bdresser]

Designs here

cc @whymarrh