E914-Refactoring and correcting the single table inheritance implemented for Response and ResponseMap classes

1.      The dependency on ResponseMap has been removed from the code i.e. every class inheriting from ResponseMap is in turn inheriting from Response class now. This doesn’t change the database hierarchy as ResponseMap and Response were using Single Table Inheritance, hence there is only one Table.
2.      The code dependency that existed on a response_map for every response (review) has been removed from the code.
3.      The reference to map_id which existed as a primary key for ResponseMap Table before single table inheritance has been removed. A Database migration has been written to remove the attribute from the table. Now the primary key  (response) id is used in place of map_id
4.     The values in the type column in the Response table have been changed to FeedbackResponse, TeamReviewResponse and MetaReviewResponse.