Previously, the reviewable insights metadata was not being serialized
correctly for admin users that flagged their own posts. There was a
conflict with the order of the `has_one` associations in the
`ReviewableSerializer`. When an admin flagged their own post, the user
was identical for both the `target_created_by` and `created_by`
attributes. This silently dropped the `target_created_by` attribute,
resulting in broken metadata (join date, post count) for the flagged post.
This commit reordered the `has_one` associations in the
`ReviewableSerializer` to ensure that `target_created_by` is serialized
before `created_by`.
0fb15b0223
