Add alert on hover when on private GitHub repo

Created by: AlicjaSuska

Part of RFC 221: Improve discoverability/awareness of the browser extension

Description Users naturally try to hover over the code in their private GitHub repositories to use Sourcegraph tooltips. Information in this context should be provided. Add alert when the user is viewing the private code.

Design documentation See in Figma

• displayed when user hovers over the private code on GitHub repo that is not connected to Sourcegraph
• 'Show more info' button links to Docs
• After dismissing the alert (click on ‘Dismiss’ or ‘x’), it is never shown again