Visual Editor - New, streamlined auth flow

[bttf]

This improves the initial auth flow for when users open the Visual Editor for the first time. This PR gets rid of the copy/paste of API credentials step for a completely automated, no-friction flow.

See accompanying PR for more details: https://github.com/growthbook/growthbook/pull/1493

Changes

• Clean up and add new Message types to facilitate API transmission
  • OpenVisualEditorRequestMessage / OpenVisualEditorResponseMessage
• Fixes options menu styling
• Clean up around legacy API token flow

Screen capture

With no prior API key stored in chrome ext. storage

Message for when the API key is not transmitted by GB app for whatever reason (e.g. self-hosted app out of date)