Demo of error page translation

This commit demonstrates front-end-side translation of an error page
for a URL like /viewer#INVALIDBOOK/whatever (where INVALIDBOOK should
be a book name NOT present in the library).

Known issues:

- This change breaks a couple of subtests in the
  ServerTest.Http404HtmlError unit test.

- Changing the UI language while an error page is displayed in the
  viewer doesn't retranslate it.

src/server/internalServer.cpp

@@ -1133,7 +1133,7 @@ std::unique_ptr<Response> InternalServer::handle_content(const RequestContext& r
 
   if (archive == nullptr) {
     const std::string searchURL = m_root + "/search?pattern=" + kiwix::urlEncode(pattern);
-    return UrlNotFoundResponse(request)
+    return UrlNotFoundResponse(request, true)
            + suggestSearchMsg(searchURL, kiwix::urlDecode(pattern));
   }
 

src/server/response.cpp

@@ -334,16 +334,20 @@ HTTPErrorResponse::HTTPErrorResponse(const RequestContext& request,
   });
 }
 
-HTTP404Response::HTTP404Response(const RequestContext& request)
+HTTP404Response::HTTP404Response(const RequestContext& request,
+                                 bool includeKiwixResponseData)
   : HTTPErrorResponse(request,
                       MHD_HTTP_NOT_FOUND,
                       "404-page-title",
-                      "404-page-heading")
+                      "404-page-heading",
+                      std::string(),
+                      includeKiwixResponseData)
 {
 }
 
-UrlNotFoundResponse::UrlNotFoundResponse(const RequestContext& request)
-    : HTTP404Response(request)
+UrlNotFoundResponse::UrlNotFoundResponse(const RequestContext& request,
+                                         bool includeKiwixResponseData)
+    : HTTP404Response(request, includeKiwixResponseData)
 {
   const std::string requestUrl = urlDecode(m_request.get_full_url(), false);
   *this += ParameterizedMessage("url-not-found", {{"url", requestUrl}});

src/server/response.h

@@ -160,12 +160,14 @@ struct HTTPErrorResponse : ContentResponseBlueprint
 
 struct HTTP404Response : HTTPErrorResponse
 {
-  explicit HTTP404Response(const RequestContext& request);
+  explicit HTTP404Response(const RequestContext& request,
+                           bool includeKiwixResponseData = false);
 };
 
 struct UrlNotFoundResponse : HTTP404Response
 {
-  explicit UrlNotFoundResponse(const RequestContext& request);
+  explicit UrlNotFoundResponse(const RequestContext& request,
+                               bool includeKiwixResponseData = false);
 };
 
 struct HTTP400Response : HTTPErrorResponse