source/scss/main.scss: Add style to blockquote tags

Bootstrap's default only styles blockquote tags that also have the
class "blockquote". As Hugo's Markdown syntax for blockquotes only
produces bare blockquote tags, we should apply the same style to
those as well, as this is most likely what users expect.

source/scss/main.scss

@@ -170,6 +170,15 @@ section + #disqus_thread {
   margin-top: 1rem;
 }
 
+// style all blockquotes, not just Bootstrap's default of class="blockquote"
+// see: node_modules/bootstrap/scss/_type.scss
+article blockquote {
+  padding: ($spacer / 2) $spacer;
+  margin-bottom: $spacer;
+  font-size: $blockquote-font-size;
+  border-left: $blockquote-border-width solid $blockquote-border-color;
+}
+
 /*
  * Footer
  */