Allows users to select browser for authentication

[gingi]

• Main app window now opens without prior authentication
• New welcome screen prompts user to sign in, while hiding side menu items and allowing the user to access settings
• An overlay prompts the user to authenticate against each tenant that requires interactive authentication, and includes a toggle for using the external (system) browser
• App settings now have ability to toggle external browser for auth
• Upgrades Angular to 11.2.12

Fixes #2446