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.

db/migrate/20200401172023_create_published_pages.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class CreatePublishedPages < ActiveRecord::Migration[6.0]
+  def change
+    create_table :published_pages do |t|
+      t.references :topic, null: false, index: { unique: true }
+      t.string :slug, null: false
+      t.timestamps
+    end
+
+    add_index :published_pages, :slug, unique: true
+  end
+end