Feature/7675 password challenge

[aerych]

Refs #7675
This PR wires up the password challenge step shown when a user attempts to login via google but a matching wpcom account has not yet been connected.

Prep:

• Create a Google account.
• Create a wpcom account with the google account's email address.

To test:

• Via a browser, log into wpcom and view your account settings page. Choose the Social Login tab and confirm the account is not connected.
• On the email login screen, tap to log in via Google.
• Authenticate with the Google account you created.
• Confirm the password screen in shown and shows the text from the mock up.
• Authenticate to wpcom.
• Via a browser, log into wpcom and view your account settings page. Choose the Social Login tab and confirm the account is connected.

Confirm that all tests pass.

Needs review: @nheagy

[nheagy]

Great!

[aerych]

Thanks Nate!!!