FEATURE: SiteSetting for creation of small action on tag change (#20812)

This adds a SiteSetting, which when enabled, creates a small_action post for tag/category changes to the topic. It uses `topic.add_moderator_post, and passes raw text in, to describe the change.

spec/lib/post_revisor_spec.rb

@@ -146,6 +146,76 @@ RSpec.describe PostRevisor do
 
       expect { subject.revise!(admin, tags: ["new-tag"]) }.not_to change { Notification.count }
     end
+
+    it "doesn't create a small_action post when create_post_for_category_and_tag_changes is false" do
+      SiteSetting.create_post_for_category_and_tag_changes = false
+
+      expect { subject.revise!(admin, tags: ["new-tag"]) }.not_to change { Post.count }
+    end
+
+    describe "when `create_post_for_category_and_tag_changes` site setting is enabled" do
+      fab!(:tag1) { Fabricate(:tag, name: "First tag") }
+      fab!(:tag2) { Fabricate(:tag, name: "Second tag") }
+
+      before { SiteSetting.create_post_for_category_and_tag_changes = true }
+
+      it "Creates a small_action post with correct translation when both adding and removing tags" do
+        post.topic.update!(tags: [tag1])
+
+        expect { subject.revise!(admin, tags: [tag2.name]) }.to change {
+          Post.where(topic_id: post.topic_id, action_code: "tags_changed").count
+        }.by(1)
+
+        expect(post.topic.ordered_posts.last.raw).to eq(
+          I18n.t(
+            "topic_tag_changed.added_and_removed",
+            added: "##{tag2.name}",
+            removed: "##{tag1.name}",
+          ),
+        )
+      end
+
+      it "Creates a small_action post with correct translation when adding tags" do
+        post.topic.update!(tags: [])
+
+        expect { subject.revise!(admin, tags: [tag1.name]) }.to change {
+          Post.where(topic_id: post.topic_id, action_code: "tags_changed").count
+        }.by(1)
+
+        expect(post.topic.ordered_posts.last.raw).to eq(
+          I18n.t("topic_tag_changed.added", added: "##{tag1.name}"),
+        )
+      end
+
+      it "Creates a small_action post with correct translation when removing tags" do
+        post.topic.update!(tags: [tag1, tag2])
+
+        expect { subject.revise!(admin, tags: []) }.to change {
+          Post.where(topic_id: post.topic_id, action_code: "tags_changed").count
+        }.by(1)
+
+        expect(post.topic.ordered_posts.last.raw).to eq(
+          I18n.t("topic_tag_changed.removed", removed: "##{tag1.name}, ##{tag2.name}"),
+        )
+      end
+
+      it "Creates a small_action post when category is changed" do
+        current_category = post.topic.category
+        category = Fabricate(:category)
+
+        expect { subject.revise!(admin, category_id: category.id) }.to change {
+          Post.where(topic_id: post.topic_id, action_code: "category_changed").count
+        }.by(1)
+
+        expect(post.topic.ordered_posts.last.raw).to eq(
+          I18n.t(
+            "topic_category_changed",
+            to: "##{category.slug}",
+            from: "##{current_category.slug}",
+          ),
+        )
+      end
+    end
   end
 
   describe "revise wiki" do