#3004905 Jetpack Remote Install: Update copy and style on remote credentials page

Changes proposed in this Pull Request

• Updates the copy and styles on the remote install self hosted credentials page.

Before:

After:

Testing instructions

• Start Calypso npm start
• Visit http://calypso.localhost:3000/jetpack/connect/install
• Type in a wordpress site such as https://cool-beaver.jurassic.ninja
• Notice that there are no errors on that page and you can proceed to the nest step.
• Type in a wrong username and password. The error message is what you expected.

Fixes #40091