Currently, target_id which refers to the ID
of the target of an auditable event is stored
within a hash. This makes querying and filtering
expensive and difficult. It also makes type
enforcement much more complicated.

This commit adds a new column (target_id)
to audit_events and writes the value to both
the old details hash and the new database column
in parallel.

It also refactors all paths within the application
that write to this value.