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/controllers/reviewable_claimed_topics_controller.rb

@@ -0,0 +1,20 @@
+class ReviewableClaimedTopicsController < ApplicationController
+  requires_login
+
+  def create
+    topic = Topic.find_by(id: params[:reviewable_claimed_topic][:topic_id])
+    guardian.ensure_can_claim_reviewable_topic!(topic)
+    ReviewableClaimedTopic.create!(user_id: current_user.id, topic_id: topic.id)
+    render json: success_json
+  end
+
+  def destroy
+    topic = Topic.find_by(id: params[:id])
+    raise Discourse::NotFound if topic.blank?
+
+    guardian.ensure_can_claim_reviewable_topic!(topic)
+    ReviewableClaimedTopic.where(topic_id: topic.id).delete_all
+
+    render json: success_json
+  end
+end