RFC-459 Browser extension fallback Sourcegraph URL

Created by: erzhtor

• RFC-459 Docs
• Figma designs

Closes https://github.com/sourcegraph/sourcegraph/issues/643

TL;DR

• Some Bext users use self-hosted Sourcegraph instances for browsing corporate/private repositories. However, they have to regularly change Sourcegraph URL when browsing private or public repo
• Also, some users don't want any requests to be sent to Sourcegraph Cloud for certain repositories (for security reasons)