Feature: Add explanatory text to the Heading widget

[caaster]

This feature will make it easier for exhibit admins to view their Home, Feature, and About pages in edit mode and know when the heading widget is being used, and be informed that it produces an H2 heading. This will create improved exhibit-building transparency for accessibility.

When the Heading widget has been used on a page and an exhibit admin looks at that page in edit mode, the widget should display the following text, with a horizontal line under the text as a visual separator:

This widget produces an H2 heading.

When an exhibit admin uses the Heading widget on a page, they are able to see the above mentioned text displayed at the top of the widget when in page edit mode.

[corylown]

We think there are more issues to consider around headings in Spotlight. This needs consideration and design.

Relates to: https://github.com/projectblacklight/spotlight/issues/3473