make OAUTH2_REDIRECT VAR optional

[mattkrick]

AC

• /:provider route no longer needs a new provider, they can share it
• OAUTH2_REDIRECT should not be necessary unless using a forwarding service
  • cloud SaaS and PPMIs use this
  • self-hosted does not (they must set up their own atlassian, slack, etc. integrations)
  • document the new process (Document how to OAUTH2_REDIRECT works and its relation with the integrations #10438)