Admins can choose to add Matrix Integration. 

This will send notifications to the registered matrix room 

about new review requets and updates to existing ones.

Tested in RB4.

Edited Slack unit test cases to test Matrix integration. 

All tests have passed except 2 of then complaining about 

missing image attachment 
tests with errors are:

- test_notify_new_review_request_with_image_file_attachment

- test_notify_updated_review_request_with_new_image_attachments