There's a problem that Slack login is required for Slack integration, but sometimes we want to use OIDC for login: this will make the application vomit.
Steps to fix it:

1. Get a Postgres connector, connect to DB. Backup DB.

2. Consider the Email domain you will use with OIDC: blah [at] cnbeining.com is cnbeining.com, test@gmail.com is gmail.com.

3. Fill in all environment variables required for OIDC integration. Remove ALLOWED_DOMAIN variable - otherwise Outline will not allow logins from Email whose domain is different from it's own domain.

4. Create a new entry in table authentication_providers. name is always oidc, domain is the domain of your Email, enabled is true, teamid is the same as the original one. Note self hosted Outline can only have 1 team at a time.

5. Go to table teams. The name is ALWAYS HARDCODED as Wiki. domain is empty. Failing to do so will cause Outline to complain "max number of teams reached".

6. Now your new OIDC should be working but users is not be associated across authentication providers by Email.

7. In user_authentications table, create a second entry for users you want to associate: random id, same userId, set authenticationProviderId, scopes and providerId as the new one's. Delete the newly created but disassociated user.

8. Now you should have more than 1 login method with users associated across the board by Email.