Backend translates the search results page

Now the search results page is presented by the backend in the language
controlled by the value of the `userlang` URL query parameter (or, if
the latter is missing, the value of the `Accept-Language:` HTTP header).

Note that the front-end doesn't yet take advantage of this
functionality.

include/search_renderer.h

@@ -72,6 +72,13 @@ class SearchRenderer
     this->pageLength  = pageLength;
   }
 
+  /**
+   * set user language
+   */
+  void setUserLang(const std::string& lang){
+    this->userlang  = lang;
+  }
+
   /**
    * Generate the html page with the resutls of the search.
    *
@@ -105,6 +112,7 @@ class SearchRenderer
   unsigned int pageLength;
   unsigned int estimatedResultCount;
   unsigned int resultStart;
+  std::string userlang = "en";
 };