Fix reviews import and GitHub Import E2E

What does this MR do and why?

This change fixes importing reviewed events. GitHub doesn't use the actor attribute for these events in their API response and instead uses user. Because of that, reviewed events were mapped to the user that triggered the migration.

Also, this change updates GitHub Import E2E to be compatible with the changes introduced by Import pull request and issues nested resources... (#433536 - closed). For context, the tests started to fail because the github_import_extended_events feature flag was enabled by default.

FF rollout issue: #435089 (closed)

Related to: #433536 (closed)

Fixes: #439703 (closed) and #439702 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Enable github_import_extended_events feature flag
2. Use the command below to trigger a migration

curl --location 'http://gdk.test:3000/api/v4/import/github' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer GDK_ACCESS_TOKEN' \
--data '{
    "personal_access_token": "GITHUB_ACCESS_TOKEN",
    "repo_id": "466994992",
    "target_namespace": "root",
    "new_name": "import-test",
    "optional_stages": {
      "attachments_import": false,
      "collaborators_import": false
    }
}'

3. Check if everything was migrated as expected