Remove global sourcegraphUrl

Created by: lguychard

Refactors the browser extension code to either inject the sourcegraphURL, or observe it directly from sync storage / local storage / window.SOURCEGRAPH_URL.

Test Plan

Code Hosts

• GitHub
• GitHub Enterprise
• Refined GitHub
• Phabricator
• Phabricator integration
• Bitbucket
• Gitlab

Browsers

• Chrome
• Firefox