Gutenberg: make sure Largo frontend post-content-specific classes are loaded in editor

[benlk]

• recheck that all post-content styles are included the editor-styles CSS file's includes
• maybe break post-content styles out into a separate stylesheet that can be included on posts/pages and as an editor stylesheet? no; post-content styles affect excerpts and custom post types; we'd need to enqueue this sheet on all is_single() (doable) and on the homepages and on indexes and on places with widgets: all places where we already enqueue style.css. Also, editor-style.less already pulls in the posts-and-pages less: https://github.com/INN/largo/blob/0.5/less/editor-style.less

[benlk]

For one dev's process of doing this, see https://wptavern.com/hemingway-theme-adds-gutenberg-support.

We should think about a post-merge blog post talking about the changes we made, a sort of cleaned-up oral history version of the changelog.