Slack Integration workflow now uses OAuth 2.0.

Users fill out the form with the credentials provided

by Slack App and clicks on a button Authorize to bring

up the OAuth 2.0 popup workflow.

Manually tested adding Slack integration using OAuth 2.0 workflow

Access token is successfully saved into config
Slack integration successfully sends notifications to

   linked channel 

  with a test Slack App set up


Ran unit tests, all works except 2 needing image attachments:

test_notify_new_review_request_with_image_file_attachment
test_notify_updated_review_request_with_new_image_attachments