audit_trail should always have an audit_id for grouping

[cornernote]

Use case:

I want to not insert audit_entry rows, unless there is errors or trails. In the case of errors or trails they should be given an audit_id for grouping and logging purposes. This means the audit_trail.user_id field is no longer required.

The problem:

Some people want to use audit_trail stand-alone, without writing tons of data to the audit_entry and audit_data tables.

The solution:

The audit_entry table should be ultra light - no large data at all. #46 should take care of this. Once it's closed we can force an audit_entry to be created when trails or errors happen, eg:

In AuditingBehavior.php change this:

$entry = Auditing::current() ? Auditing::current()->getEntry() : null;

to this:

$entry = Yii::$app->auditing->getEntry(true);