Listing variations with summary component

[davisagli]

What's included here:

• Re-label the built-in listing variations to List (was Default), List with images (was Summary), and Grid.
• Add a new Summary component which is looked up by content type and renders the kicker, title, and description
• CSS tweaks to make the kicker look correct where it was not shown before
• Add Summary component implementations for News Item, Event, and File
• Adjust date rendering in EventView, NewsItemView, and EventMetadata block for better consistency
• Docs

[sneridagh]

LGTM, but look at the comments, specially for the introduction of date-fns. Let's talk about it.

[sneridagh]

One of the things that I discussed with @stevepiercy several times is to standardize glossary and concepts. We haven't settled yet on one, but I'd call this "default configuration loader" and try to be consistent everywhere.

[davisagli]

@sneridagh I'm happy to update it once there's a glossary entry I can link to.

[sneridagh]

@stevepiercy how could we do it? More than words, are even concepts... like in this case. "Every time we refer to the applyConfig / add-on's default exported function, etc we say instead: default configuration loader"