handlers: permisison check for all pages + retrieve user-specific permissions for pages (if any)

handlers/handlerpageview.h

@@ -9,10 +9,7 @@ class HandlerPageView : public HandlerPage
 {
   protected:
 	bool canAccess(std::string page) override;
-	std::string accessErrorMessage() override
-	{
-		return "You don't have permission to view this page";
-	}
+	std::string accessErrorMessage() override;
 	std::string createIndexContent(IParser &parser, std::string content);
 
   public: