Enhance OneSignal widget to reflect signup status

[davisshaver]

New JS approach based on official docs: https://documentation.onesignal.com/docs/web-push-custom-code-examples#section-custom-link-permission-message

• Allows for multiple widgets on page
• Provides new "loading" message while subscription status retrieved
• Changes message/action based on subscription status
• Allows unsubscription message to be changed with widget
• Uses textwidget class for better default styling

This change is 

[rgomezp]

Thanks so much for your work on this! Let us know as soon as the PR is ready for review and we'll try to review asap

[davisshaver]

Thank you @rgomezp, this is ready for review. I have updated the branch to include the recent changes for VIP.

[rgomezp]

The reason this hasn't been merged is due to limited bandwidth and because it still requires testing and approval by WP VIP team.

I don't want to close it so that it doesn't get lost