Fix proxy to redirect to host and port set by cli

[mgmanzella]

WHY are these changes introduced?

Fixes #2606

WHAT is this pull request doing?

Use the host and port set by the cli to replace URLs in the proxy. Before it defaulted to 127.0.0.1:9292 but this isn't correct for extension serve

How to test your changes?

Testing steps assume you have a theme app extension (TAE) created

• Create a new dev store
• Run extension serve in your TAE
• In your browser go to http://127.0.0.1:8282
• Confirm you're redirected to the store password page
• Input your password and confirm you can get into the store

fix_redirect_proxy.mp4