Fix background style in investments search summary

The background wasn't expanding to the edge of the page because we
forgot to do this when we did the same thing for proposals and debates
in commit 4c47eab60.

app/views/budgets/investments/index.html.erb

@@ -22,7 +22,7 @@
   <% end %>
 
   <% if params[:search].present? || params[:advanced_search].present? %>
-    <div class="highlight no-margin-top padding margin-bottom">
+    <div class="search-results-summary">
       <div class="row">
         <div class="small-12 column">
           <h2><%= t("shared.search_results") %></h2>