Make SessionStore a proper singleton

- SessionStore class now has a proper singleton structure
- session_storage_singleton.hpp is removed
- from_json(..) function for SessionStore is changed to a specialized
template
- minor cosmetic fixes added
- Move the template class usages of Crow App over to a non-template 
parameter


Change-Id: Ic9effd5b7bac089a84c80a0caa97bd46d4984416
Signed-off-by: Borawski.Lukasz <lukasz.borawski@intel.com>
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/include/webserver_common.hpp b/include/webserver_common.hpp
index 1ba091b..4d88629 100644
--- a/include/webserver_common.hpp
+++ b/include/webserver_common.hpp
@@ -15,6 +15,7 @@
 */
 #pragma once
 
+#include "security_headers_middleware.hpp"
 #include "token_authorization_middleware.hpp"
 #include "security_headers_middleware.hpp"
 #include "webserver_common.hpp"
@@ -22,4 +23,3 @@
 using CrowApp = crow::App<crow::PersistentData::Middleware,
                           crow::TokenAuthorization::Middleware,
                           crow::SecurityHeadersMiddleware>;
-