Add preview field to YAML front matter for posts

This is a step toward issue #34. Currently, only issue 1 of volume 2
has the preview text, but eventually all past issues should have it.

Author: dellsystem

_includes/head.html

@@ -38,7 +38,7 @@
 
     {% if page.title %}
     <meta property="og:title" content="{{ page.title }}" />
-    <meta property="og:description" content="{{ page.excerpt | normalize_whitespace | strip_html | truncatewords: 40 }}"/>
+    <meta property="og:description" content="{% if page.preview %}{{ page.preview }}{% else %}{{ page.excerpt | normalize_whitespace | strip_html | truncatewords: 40 }}{% endif %}"/>
     {% else %}
     <meta property="og:title" content="{{ site.title }}" />
     <meta property="og:description" content="{{ site.description }}" />

_posts/2020-05-01-issue-1.md

@@ -8,6 +8,7 @@ image:
     caption: Chris Smalls and essential workers talking the talk, walking the walk out.
     source_link: https://twitter.com/Shut_downAmazon/status/1255681771292721158
     half_width: false
+preview: "The TWC newsletter is back, with a special May Day edition!"
 ---
 
 Today is May Day, International Workers Day, and [#EssentialWorkersDay](https://twitter.com/search?q=%23EssentialWorkersDay). As the coronavirus pandemic continues, it becomes increasingly clear how much our economy depends on the workers who have been deemed “essential”. Yet these workers are rarely given the protective equipment they need — not to mention decent wages or benefits — proving that “essential” really means “sacrificial”. Chris Smalls, an Amazon warehouse worker and organizer fired last month, shared a [video montage](https://twitter.com/Shut_downAmazon/status/1255681771292721158) with dozens of essential workers protesting for better working conditions at Instacart, Shipt, Amazon warehouses, and your neighborhood Whole Foods. Today, they are on strike with a [list of demands](https://docs.google.com/document/d/1KD8uaVch14xBD1TTZ-TKbAub-IcqERS6O1OVeL2HbtY/edit).