Fix JWT token authentication for RESTful API endpoints

[sreekanthreddybalne]

Issue

Regression introduced in commit https://github.com/djangoflow/django-df-chat/commit/6ca78e273ef4286a77fe9a525e479b93daf66ca5

• The REST_FRAMEWORK config was removed from the django settings.
• This caused the JWT token authentication to fail.

Due to this the /api/v1/chat/<room_pk>/messages/ endpoint is returning a 403 Forbidden error response.

Fix

• Added the REST_FRAMEWORK config to django settings.
• Created a "BaseTestUtilsMixin", which holds the following methods:
  • create_user and create_room_and_add_users, to be used by synchronous test methods.
  • async_create_user and async_create_room_and_add_users, to be used by asynchronous test methods. Mostly used in the testing of websocket communication.
• Added unit test to test creation of message using the messages endpoint.