Add code to user migration script to label users origin/auth strategy

[rhysyngsun]

A management command was recently added that performs an export of user data to Keycloak. We want to preserve some information about where the user's account was first migrated from and what authentication strategy (touchstone, email+pass) they used so that we can intelligently navigate them through flows in Keycloak.

Acceptance Criteria

• Exported user accounts are labeled as originating from discussions.
• Exported user accounts are labeled as having had touchstone, email, and/or micromasters as their authentication strategy.

[collinpreston]

Completed in 4249 using Keycloak group assignment instead of labels.