Implement dropdown for ease of Sourcegraph instance URL switching to browser extension

Created by: jjinnii

After reviewing the complexity and privacy implications of implementing fallback logic in RFC 542, the decision was made to not implement fallback logic and instead move forward with a dropdown solution that alleviates the user pain point of switching between multiple instances.

Figma Designs

UI Updates

• Implement new dropdown feature
• Re-skin browser extension to match new Wildcard styling

Metrics

• Explore possibility and security implementations of tracking the # of users who have more than 1 URL stored in browser extension storage. This will be a very helpful metric for the team in the future as a base number of our bext user base that uses more than Sourcegraph instance.