Move theme options into the customizer

[kjellr]

In order to align to TRT guidelines, we should move the Gutenberg Starter theme options out of a custom admin page and into the Customizer. This PR migrates these settings over.

Testing Instructions

1. Install the PR version of this theme on a test site, do not change any settings yet.
2. Ensure that the options above are all active by default:
   • You should see wide and full alignment options available in the editor.
   • You should see the custom theme color palette available in the editor.
   • You should see light mode active, not dark mode.
   • You should see default block styles applied on the frontend. (A quick way to check is to verify that the quote block appears with a left border).
   • On the front end, the body should have a wp-embed-responsive class.
3. Check and uncheck the new and verify that they toggle the options as expected.

Screenshots

Before:

After:

[kjellr]

Closing due to inactivity. 🕐