BE [Group Owner bypass] Skip confirmation when allow bypass of placeholder user confirmation is enabled

About

This is part of the epic &17871. See that epic for more context.

Implementation

⚠️ All MRs must be reviewed by AppSec (@ameyadarshan).

This issue follows from the work implemented in #534329 (closed) and should be behind a feature flag importer_user_mapping_allow_bypass_of_confirmation

backend must make correct checks before allowing the bypass.

All of these must be true for us to allow the by-pass:

• Feature flag (importer_user_mapping_allow_bypass_of_confirmation) is enabled
• Group has the setting added in #534329 (closed) enabled
• Group has correct license tier
• User being reassigned is one of the group's enterprise users
• It's GitLab.com
• Group is a top-level group.

This is a combination of checking our feature flag is enabled, the setting added in #534329 (closed) is enabled, and User#managed_by_group?(group) which takes care of the other checks.

We must validate in both:

• the service/controller layer and
• also within the model - see the implementation of admin bypass !190272 (merged)

The logic should be encapsulated in a class in Import::UserMapping that resembles the admin by-pass class #542284 (comment 2513379053).

Note a follow-up issue BE [Group Owner bypass ] Implement "enterprise ... (#540573 - closed) will add an "expiry" time to the check later.