Cells: Iteration 2: Configure `authentication_events` to be `gitlab_main_clusterwide` to support `User can create Group`

This table authentication_events is written to while trying to execute workflow User can create group. See !126635 (comment 1494660726)

Proposal

1. Investigate why user_id is allowed to be null. We need to know if it is reasonable for this table to be gitlab_main_clusterwide
2. Assign authentication_events to gitlab_main_clusterwide.
3. Identify all cross-joins violations and define which ones should be solved to support this workflow.
4. app/models/authentication_event.rb should use < MainClusterwide::ApplicationRecord