Instagram Widget: Disable "Connect" button if widget is not published

#### Steps to reproduce the issue

1. Go to `/wp-admin/customize.php?autofocus[panel]=widgets`
2. Add Instagram widget
3. Note that "Connect" button is active
4. Also, note the "Important" text above the button.

#### What I expected

"Connect" button is disabled until the widget is not published

#### What happened instead

"Connect" button is active when widget is not published. "Important" note is not very well visible and could be easily missed

**Screenshots**

Reproduce the issue at /wp-admin/customize.php?autofocus[panel]=widgets by adding the Instagram widget and checking the Connect button and Important note. Locate the widget implementation and its existing tests, then make the button remain disabled until the widget is published and ensure the warning is visible; verify the unpublished and published states.

Written by the indexing model from the issue text.