FEATURE: Support for publishing topics as pages (#9364)

If the feature is enabled, staff members can construct a URL and publish a
topic for others to browse without the regular Discourse chrome.

This is useful if you want to use Discourse like a CMS and publish
topics as articles, which can then be embedded into other systems.

app/controllers/published_pages_controller.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+class PublishedPagesController < ApplicationController
+
+  skip_before_action :preload_json
+  skip_before_action :check_xhr, :verify_authenticity_token, only: [:show]
+  before_action :ensure_publish_enabled
+
+  def show
+    params.require(:slug)
+
+    pp = PublishedPage.find_by(slug: params[:slug])
+    raise Discourse::NotFound unless pp
+
+    guardian.ensure_can_see!(pp.topic)
+    @topic = pp.topic
+    @canonical_url = @topic.url
+    render layout: 'publish'
+  end
+
+  def details
+    pp = PublishedPage.find_by(topic: fetch_topic)
+    raise Discourse::NotFound if pp.blank?
+    render_serialized(pp, PublishedPageSerializer)
+  end
+
+  def upsert
+    result, pp = PublishedPage.publish!(current_user, fetch_topic, params[:published_page][:slug].strip)
+    json_result(pp, serializer: PublishedPageSerializer) { result }
+  end
+
+  def destroy
+    PublishedPage.unpublish!(current_user, fetch_topic)
+    render json: success_json
+  end
+
+  def check_slug
+    pp = PublishedPage.new(topic: Topic.new, slug: params[:slug].strip)
+
+    if pp.valid?
+      render json: { valid_slug: true }
+    else
+      render json: { valid_slug: false, reason: pp.errors.full_messages.first }
+    end
+  end
+
+private
+
+  def fetch_topic
+    topic = Topic.find_by(id: params[:topic_id])
+    guardian.ensure_can_publish_page!(topic)
+    topic
+  end
+
+  def ensure_publish_enabled
+    raise Discourse::NotFound unless SiteSetting.enable_page_publishing?
+  end
+
+end