[Task]: ensure logging or tracking of 2nd level approval

[mixedpuppy]

Description

2nd level approvals need to be fully auditable. Currently we do not log or track who makes those decisions.

https://github.com/mozilla/addons-server/blob/c7adcf372770c4dd60c5954a5f5c0cec89a8be61/src/olympia/reviewers/views.py#L1287

The YES condition doesn't pass the user up for any potential logging. The other conditions do, but I am not certain on how easily auditable this is.

How the user at this stage is tracked should be consistent across these decisions.

Acceptance Criteria

• logging or user field, consistent across YES/NO/CANCEL

┆Issue is synchronized with this Jira Task

[diox]

We can probably add approver_user to the ContentDecision or something like that, or simply emit an extra ActivityLog.