FEATURE: add after-reviewable-post-user plugin outlet (PR #9397)

Add a plugin outlet after reviewable post user

Additionally adds custom field user serializer

Add a basic user serializer that includes custom fields. Allows review queue serializer to include custom fields for its users


I’ll merge this tomorrow - this is to allow staff notes to appear in the review queue like it did in the old flag queue.

One thing that’s confusing here is we already have a model called UserCustomField and that means this would be the default serializer for that model.

Could we use a name like UserWithCustomFields instead to differentiate?

Oh, that’s a good one - can do! Naming is hard.