Slack import conflicts with externally-managed users

[ThiefMaster]

Summary

Importing from Slack creates users, with username/email combos that might conflict with data from SSO.

Steps to reproduce

• Enable SSO (e.g. GitLab)
• Have a user in SSO with username foo and email foo@example.com
• Have a user in Slack with username foo and email bar@example.com

Expected behavior

The slack-imported user gets a custom prefix/suffix to distinguish from SSO users, e.g. foo_slack

Observed behavior (that appears unintentional)

Assuming foo from SSO never logged in, a new user foo is created with foo@example.com during the Slack import.
Now if foo logs in via SSO, he gets a username different from the SSO-provided username (iirc. foo0).

Possible fixes

• Easy workaround: Add an option to restrict Slack import to System Admins (that way the impact is greatly reduced since not every team admin can do an import that creates conflicting users)
• Clean solution: Add an option to provide a prefix/suffix for users created during Slack import
• (EE+LDAP) Match emails and update usernames from Slack with the matching usernames from SSO (prefix/suffix if no match) during import.

Related thread: https://community.mattermost.com/core/pl/3nmcmx3jz3815mff1fcnbi7u9e

[amyblais]

Thank you @ThiefMaster, opened a bug ticket here: https://mattermost.atlassian.net/browse/MM-15748.