Add a new tab page for private tabs

[bbondy]

Test plan

See #8074

Description

Since cc51723 we have the ability to render new tab pages for private tabs.

js/about/newtab.js can do this here:

   if (window.chrome.extension.inIncognitoContext) {
    return <div className='empty' />
  }

Please display a page that @bradleyrichter specs out.

Side note about the implementation, please move window.chrome.extension.inIncognitoContext outside of the component to make the component more unit testable without mocking. I.e. pass it in as props.

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@bradleyrichter, I like Firefox's new private tab page, in particular props for this line:

Private Browsing doesn’t make you anonymous on the Internet. Your employer or Internet service provider can still know what page you visit.

And also that they say what will be stored and what won't explicitly. The same applies to us.

No controls for this round please (like controlling tracking protection like firefox has). Only a nice looking page with info.