Generalized Review System - Milestone 1.1 (Part 3): Suggestion servic…

…es (oppia#4982)

* added suggestion services

* fixed linting

* minor fixes

* minor fixes

* added comments above lines that will be edited

* removed unnecessary print

* made sub class for types of suggestion and added validate()

* added get_model_corresponding_to_suggestion function and tests, linting fixes

* added docstring

* simplified test

* added accept and from_dict functions

* added not implemented checks

* made more comprehensive validators

* fixed linting

* review changes

* minor fix

* linting fixes

* linting fixes

* made review changes

* minor fix

* minor fix to validators

* linting fix

* review changes

* minor fixes

* fixed linting

* made review changes

* made review changes

* more review changes

* more review changes

* review changes (1)

* linting fixes

* more linting fixes

* minor fix

* fix backend tests

* added basic integration test

* linting fixes

* more linting fixes

* added test for rejection and accept with message

* made review changes

* linting fix