FEATURE: Claim Reviewables by Topic

This is a feature that used to be present in discourse-assign but is
much easier to implement in core. It also allows a topic to be assigned
without it claiming for review and vice versa and allows it to work with
category group reviewers.

app/models/reviewable_user.rb

@@ -101,7 +101,6 @@ end
 #  created_by_id           :integer          not null
 #  reviewable_by_moderator :boolean          default(FALSE), not null
 #  reviewable_by_group_id  :integer
-#  claimed_by_id           :integer
 #  category_id             :integer
 #  topic_id                :integer
 #  score                   :float            default(0.0), not null