Add instance-wide setting to disable profile directory (#9497)

* Add instance-wide setting to disable profile directory

Fixes #9496

When the profile directory is disabled:
- The “discoverable” setting is hidden from users
- The “profile directory” link is not shown on public pages
- /explore returns 404

* Move Setting.profile_directory check to a before_action filter

app/controllers/directories_controller.rb

@@ -3,6 +3,7 @@
 class DirectoriesController < ApplicationController
   layout 'public'
 
+  before_action :check_enabled
   before_action :set_instance_presenter
   before_action :set_tag, only: :show
   before_action :set_tags
@@ -18,6 +19,10 @@ class DirectoriesController < ApplicationController
 
   private
 
+  def check_enabled
+    return not_found unless Setting.profile_directory
+  end
+
   def set_tag
     @tag = Tag.discoverable.find_by!(name: params[:id].downcase)
   end