'About Sourcegraph' and 'Help' pages should be opened in the separate tab

Created by: AlicjaSuska

Current state:

• Authorized users: 'Help' and 'About Sourcegraph' links, accessible from the 'Profile' dropdown, opens in the new tab
• Un-authorized users: 'Help' and 'About' links, accessible from the top navigation, opens in the same tab.

Proposed changes:

• Open Help and About links in the separate tab for all users. We would avoid taking them out of the context of the app.
• Add 'open in the new tab' icon in the dropdown menu visible for authorized users.

@rrhyne @felixfbecker please, review. Thank you!

See in Figma