Adds help boxes to no results page

Why are these changes being introduced:

* This information is arguably more important on no results pages than
  on regular results pages, as users may need more guidance

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/USE-280

How does this address that need:

* Adds `results_callouts` partial to no results section of display logic

Document any side effects to this change:

* This partial shows up in two places. A comment has been added to note
  this to make future changes easier reason about.

Author: JPrevost

app/assets/stylesheets/partials/_results.scss

@@ -261,6 +261,7 @@
   font-size: 1.6rem;
   line-height: $lh-xloose;
   margin: 0;
+  margin-bottom: 1em;
   max-width: 760px;
 
   a {

app/views/search/results.html.erb

@@ -31,6 +31,7 @@
                   <% end %>
                 <% end %>
             </ol>
+            <%# Note `results_callouts` is also displayed in no errors condition below %>
             <%= render partial: "results_callouts" %>
           </main>
           <%= render partial: "results_sidebar" %>
@@ -42,6 +43,9 @@
         <div id="results-layout-wrapper">
           <main id="results">
             <%= render partial: "no_results" %>
+
+            <%# Note `results_callouts` is also displayed in results and no errors condition above %>
+            <%= render partial: "results_callouts" %>
           </main>
         </div>