bext: make 'Configure on Sourcegraph' button navigate to manage repositories page when on default Sourcegraph URL

Created by: taras-yemets

Closes #3066

https://user-images.githubusercontent.com/25318659/155352705-537b6d6b-5394-4af8-ab1d-f7c041882d3d.mp4

Before	After

Test plan

• sg run bext
• SOURCEGRAPH_API_URL=https://sourcegraph.com sg start web-standalone
• replace sourcegraphURL with your local development server URL (https://sourcegraph.test:3443 on my machine) here and here
• make sure you're logged in to Sourcegraph in browser extension and local development server instance
• open private repository which is not synchronized with your Sourcegraph account and:
  • click 'Configure on Sourcegraph' button and check if you were redirected to Sourcegraph manage repositories page with repository name entered as a filter query
  • open browser extension popup and check if private repository message is there with a link to manage repositories page without any filter queries