Update query string to include theme id for development theme preview

[muriloshimizu]

Fixes #18. Also fixes an issue with the Lighthouse CI returning only 1 report (of the password page itself).

Setup:

• Using a Custom App for authentication
• All standard variables set via GitHub Secrets
• I'm not using my Partner account for this Shopify Store (I'm using a staff account instead)

The problem:

• Lighthouse CI is running tests on the live store, not on the development store.
• The Lighthouse report is only showing results for the password page of the live store.

How this PR fixes the problem: One of the main things I noticed, is that $host is the main Shopify store url, which ignores the new development store created by this action. That $host is being used on a few places where the development preview url is needed instead.

Maybe it was possible to use $preview_url instead of $host, but at the same time 1) the preview_url path differs for the index, products and collections; and 2) we need the $query_string.

Steps to fix the problem:

• When we push the development store and it returns the json, create a variable for the new theme id. - Added the theme id on the query string variable, with modifications "?preview_theme_id=$(echo "$preview_id")&_fd=0&pb=0". This will be used on the lighthouse action (around lines 172-198).
• Then also added the query string (with the theme id) on the password page url if the store is password protected (lines 206-215), to make sure we're entering the password on the correct store preview.

Other considerations:

• I only tested this on one Shopify store
• The store tested is a development Shopify Plus store with password enabled
• Tested using a fresh Dawn Theme, although the Shopify store itself is not fresh
• I'm a front-end developer and my Git, GitHub Actions, node and docker knowledge is still under development. That said, the changes worked for me, so I wanted to share with the community.

Note: this is my first ever PR for a public repo, hooray!

[muriloshimizu]

Just signed the CLA, not sure how to run the check again...