Improve user expierence with setting public github URLS

Summary

When a user decided to set a public URL for Github, the interface is a little misleading as they can change the URL in the URL field. But in actuality they should leave the URL set to the default of: https://api.github.com. By leaving this field open to edits, we allow the user to potentially get into a bad state by allowing them to set a distinct URL for the repo as they would when using Enterprise Github.

Desired Behavior

1. When a user choices to set a public github URL, ensure the field can't be edited.
2. Change the language in the first things first section to more explicit.

"Let's get you started! Set up this Nerdpack by configuring your organization's GitHub URL. If you are hosting your source code on a public Github repository you will use https://api.github.com. If you are hosting your source code on a private Github Enterprise repository provide that URL below."

3. After this is set for a user, route the user to the README.md tab in the application so it's clear the URL has been set. This will show the user the README.md has been fetched correctly.