Issue #3391489: Replace `.prose` selector, bind only once

Two changes here:

• The .prose CSS selector presumes the use of a theme which uses that class, eg Sector Radix Starterkit. We replace that with the .chunker-section selector to detect chunked content instead.
• When tested in D10 with BigPipe, Gin admin theme and toolbar and an SVG logo, the bind events were triggered repeatedly, resulting in a single "Next" click advancing several sections through the document. By using the once plugin, we can ensure a single bind of the element's click event.