FEATURE: Allow customization of robots.txt (#7884)

* FEATURE: Allow customization of robots.txt

This allows admins to customize/override the content of the robots.txt
file at /admin/customize/robots. That page is not linked to anywhere in
the UI -- admins have to manually type the URL to access that page.

* use Ember.computed.not

* Jeff feedback

* Feedback

* Remove unused import

app/controllers/admin/robots_txt_controller.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+class Admin::RobotsTxtController < Admin::AdminController
+
+  def show
+    render json: { robots_txt: current_robots_txt, overridden: @overridden }
+  end
+
+  def update
+    params.require(:robots_txt)
+    SiteSetting.overridden_robots_txt = params[:robots_txt]
+
+    render json: { robots_txt: current_robots_txt, overridden: @overridden }
+  end
+
+  def reset
+    SiteSetting.overridden_robots_txt = ""
+    render json: { robots_txt: original_robots_txt, overridden: false }
+  end
+
+  private
+
+  def current_robots_txt
+    robots_txt = SiteSetting.overridden_robots_txt.presence
+    @overridden = robots_txt.present?
+    robots_txt ||= original_robots_txt
+    robots_txt
+  end
+
+  def original_robots_txt
+    if SiteSetting.allow_index_in_robots_txt?
+      @robots_info = ::RobotsTxtController.fetch_default_robots_info
+      render_to_string "robots_txt/index"
+    else
+      render_to_string "robots_txt/no_index"
+    end
+  end
+end