Strip html and shortcodes from layout files

We have a bug where .Summary will output HTML content in rendered summaries.

We should call plainify and strip the shortcodes for our list.html template, and any other templates we may have that call .Summary.

Refer to https://gitlab.eclipse.org/eclipsefdn/it/webdev/hugo-solstice-theme/-/blob/main/layouts/_default/summary.html?ref_type=heads for how we do it in head.html