FEATURE: Show a button to Staff for "Moderation History" on posts/topics

When clicked, it pops up a modal showing a history of moderation actions
taken on the post or topic.

spec/requests/admin/moderation_history_controller_spec.rb

@@ -0,0 +1,55 @@
+require 'rails_helper'
+
+RSpec.describe Admin::BackupsController do
+  let(:admin) { Fabricate(:admin) }
+
+  before do
+    sign_in(admin)
+  end
+
+  describe "parameters" do
+    it "returns 404 without a valid filter" do
+      get "/admin/moderation_history.json"
+      expect(response).not_to be_success
+    end
+
+    it "returns 404 without a valid id" do
+      get "/admin/moderation_history.json?filter=topic"
+      expect(response).not_to be_success
+    end
+  end
+
+  describe "for a post" do
+    it "returns an empty array when the post doesn't exist" do
+      get "/admin/moderation_history.json?filter=post&post_id=99999999"
+      expect(response).to be_success
+      expect(::JSON.parse(response.body)['moderation_history']).to be_blank
+    end
+
+    it "returns a history when the post exists" do
+      p = Fabricate(:post)
+      p = Fabricate(:post, topic_id: p.topic_id)
+      PostDestroyer.new(Discourse.system_user, p).destroy
+      get "/admin/moderation_history.json?filter=post&post_id=#{p.id}"
+      expect(response).to be_success
+      expect(::JSON.parse(response.body)['moderation_history']).to be_present
+    end
+
+  end
+
+  describe "for a topic" do
+    it "returns empty history when the topic doesn't exist" do
+      get "/admin/moderation_history.json?filter=topic&topic_id=1234"
+      expect(response).to be_success
+      expect(::JSON.parse(response.body)['moderation_history']).to be_blank
+    end
+
+    it "returns a history when the topic exists" do
+      p = Fabricate(:post)
+      PostDestroyer.new(Discourse.system_user, p).destroy
+      get "/admin/moderation_history.json?filter=topic&topic_id=#{p.topic_id}"
+      expect(response).to be_success
+      expect(::JSON.parse(response.body)['moderation_history']).to be_present
+    end
+  end
+end