Allow UI rendering outside of the standard meta box

[petenelson]

On a client project, we want to add the content connect UI to a custom WooCommerce product tab. Changing render() and other methods within the MetaBox class to be static methods would allow other code to easily render the UI in different parts of the WP admin instead of just the built-in meta boxes.

[petenelson]

We also have product variations for one product (basically multiple posts in one page), with each variation needing the content connect metabox UI. The current implementation only allows for one Vue app https://github.com/10up/wp-content-connect/blob/master/assets/js/src/content-connect.js#L15

Extending this to use a localized window object targeting multiple DOM elements may allow this to be used multiple times.