Close the two remaining /gift funnel gaps

Add a hero image (rendered from the redesigned PDF's own cover) so the
opt-in page shows the actual resource instead of pure text, and make
the post-submit message PDF-specific with a working /gift-thanks
fallback link instead of a generic dead-end "check your inbox" line.

Both are opt-in per-page params (hero_image, success_message) so the
real drip-course page's rendering is unchanged.

layouts/courses/single.html

@@ -2,6 +2,7 @@
 <section class="course-hero">
   <h1>{{ .Title }}</h1>
   {{ with .Params.hero_subhead }}<p class="course-subhead">{{ . }}</p>{{ end }}
+  {{ with .Params.hero_image }}<img class="course-hero-image" src="{{ . }}" alt="">{{ end }}
 
   {{ partial "course-enroll-form.html" (dict "id" "top" "page" .) }}