Add microsite link to site home 'See also' sidebar (#1263)

Fixes #1199 

<img width="304" alt="stanford_university_libraries-2" src="https://user-images.githubusercontent.com/101482/47684252-71273000-db8f-11e8-8673-40018b3a9998.png">

app/views/shared/_site_sidebar.html.erb

@@ -1,6 +1,10 @@
 <div class="sidebar home-page-column">
   <h2 class="nav-heading">See also</h2>
   <ul class="nav sidenav">
+    <li>
+      <%= link_to "Spotlight at Stanford microsite",
+      "https://library.stanford.edu/research/spotlight" %>
+    </li>
     <li>
       <%= link_to "Other library collections at Stanford",
       "http://library.stanford.edu/collections" %>