[Setup]: Show auto-key add notification to user

[sidvishnoi]

Show a notification UI on wallet page when adding public key to wallet, to inform user of progress or failure; and not cause confusion or intimidate user as we're controlling the browser.

Part of #613

[sidvishnoi]

cc: @RabebOthmani

[sidvishnoi]

Had a discussion with @RabebOthmani today. We are inclined towards:

• Adding a consent screen before starting page takeover to add key (filed #625)
• Overlaying the page (instead of a notifcation) with a "full screen progress spinner screen" so user isn't intimidated by things clicks and navigation happening without their direct action