Implement browser extension Sourcegraph URL dropdown for ease of URL switching

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/29030. Closes https://github.com/sourcegraph/sourcegraph/issues/25778.

Description

This PR:

• Implement browser extension Sourcegraph URL dropdown for ease of URL switching
• Updates options page UI

How to test

1. sg run bext
2. Open browser extension options page
3. Check that the input field suggests all previously successfully validated URLs

Screenshots

Before	After
n/a

Before merging

• Test on different code hosts (if applicable)

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

• Test on different browsers (if applicable)

  • Chrome
  • Firefox
  • Safari

• Add change log message to client/browser/CHANGELOG.md, under "Unreleased" section. (if applicable)