[consider] Use notification to approve prompt to reduce popup annoyance

Nobody has claimed this yet.

Since `ethereum.enable()` shipped, many dapps have just slapped that line at the top of their code, which makes MetaMask pop up on page load for some pages. This is a bit of user harassment, and perhaps we can provide tools to help users protect themselves from badly-behaving sites.

One idea that came up from a conversation with `Sylvain Laurent` on ConsenSys slack is the possibility of using the web [notification api](https://developer.mozilla.org/en-US/docs/Web/API/notification) to indicate to a user that the site would like to get their input, before initiating the full popup.

The buttons could be

- `Allow` `Ignore` and `Block forever`.

This makes it easier for a user to dismiss annoying popups, improves the responsiveness time to ask a user if they want to block a site.

Curious how our designers will see something like this. @MetaMask/design

Start by reviewing the ethereum.enable flow and the linked Web Notification API proposal; the issue names no files, tests, or entry points. The scope and acceptance criteria still need a design decision covering whether and how notification controls should precede the full popup.

Written by the indexing model from the issue text.