added support for Model migrations

now we have a table core.migration and each model (User, Group, Item, ItemContent and a new Migration) have its own
row in the table with a version number

added to config:
db_make_migration_if_needed and db_stop_if_migration_fails (need description yet)

winixd/core/config.h

@@ -198,10 +198,11 @@ public:
 	std::wstring db_user;
 	std::wstring db_pass;
 
-	// is the PostgreSQL later than 10
-	// default false
-	// if true then we are not using ROW() statements in sql query
-	bool db_postgresql_smaller_than_10;
+	// make database migration if needed
+	//
+	bool db_make_migration_if_needed;
+
+	bool db_stop_if_migration_fails;
 
 	// the name of the cookie which has the session identifier
 	std::wstring http_session_id_name;