Show excerpts if they're not too long

In practice, that means we'll just copy-paste the "bylines" for
the Snipette excerpts. Then they'll be short enough ;-)

src/components/common/PostCard.js

@@ -25,7 +25,7 @@ const PostCard = ({ post, isLarge }) => {
                 {post.tags && post.tags[0] && <div className="post-card-tags">{post.tags[0].name}</div>}
                 {post.featured && <span>Featured</span>}
                 <h2 className="post-card-title">{post.title}</h2>
-                <section className="post-card-excerpt">{ /* post.subtitle */ }</section>
+                <section className="post-card-excerpt">{ post.excerpt.length <= 120 && post.excerpt }</section>
                 <footer className="post-card-footer">
                     <div className="post-card-footer-left">
                         <div className="post-card-avatar">

src/styles/app.css

@@ -862,6 +862,8 @@ h6 {
 .post-card-excerpt {
     font-size: 1.6rem;
     line-height: 1.55em;
+    color: #7F7F7F;
+    margin: 0.2rem 0.2rem 2rem 0.2rem;
 }
 
 .post-card-image {