URL-not-found message went into i18n JSON resource

Yet, the URL-not-found message is not yet fully internationalized
since its usage is hardcoded to English.

src/server/response.cpp

@@ -21,6 +21,7 @@
 #include "request_context.h"
 #include "internalServer.h"
 #include "kiwixlib-resources.h"
+#include "i18n.h"
 
 #include "tools/regexTools.h"
 #include "tools/stringTools.h"
@@ -131,8 +132,12 @@ HTTP404HtmlResponse::HTTP404HtmlResponse(const InternalServer& server,
 HTTPErrorHtmlResponse& HTTP404HtmlResponse::operator+(UrlNotFoundMsg /*unused*/)
 {
   const std::string requestUrl = m_request.get_full_url();
-  kainjow::mustache::mustache msgTmpl(R"(The requested URL "{{url}}" was not found on this server.)");
+  const auto urlNotFoundMsg = i18n::expandParameterizedString(
-  return *this + msgTmpl.render({"url", requestUrl});
+      "en", // FIXME: hardcoded language
+      "url-not-found",
+      {{"url", requestUrl}}
+  );
+  return *this + urlNotFoundMsg;
 }
 
 HTTPErrorHtmlResponse& HTTPErrorHtmlResponse::operator+(const std::string& msg)

static/i18n/en.json

@@ -5,4 +5,5 @@
 	},
 	"name":"English",
 	"suggest-full-text-search": "containing '{{{SEARCH_TERMS}}}'..."
+	, "url-not-found" : "The requested URL \"{{url}}\" was not found on this server."
 }

static/i18n/qqq.json

@@ -6,4 +6,5 @@
 	},
 	"name": "Current language to which the string is being translated to.",
 	"suggest-full-text-search": "Text appearing in the suggestion list that, when selected, runs a full text search instead of the title search"
+	, "url-not-found" : "Error text about wrong URL for an HTTP 404 error"
 }