Improve maintainability of Case Studies styling

- Add quote and lead shortcode
- Add case study metadata in front matter (to generate page)
- Allow case study page to inherit similar styles from a centralised CSS file.

layouts/shortcodes/case-studies/quote.html

@@ -0,0 +1,12 @@
+{{ if .Get "image" }}
+<div class="quote banner {{ if $.Page.Params.use_gradient_overlay }}overlay{{ end }}" style="background-image: url({{ .Get "image" }})">
+{{ else }}
+<div class="quote banner">
+{{ end }}
+  <p class="quote-text">
+    {{ .Inner }}
+    {{ if .Get "author" }}
+      <span class="quote-author">&mdash; {{ .Get "author" }}</span>
+    {{ end }}
+  </p>
+</div>