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/admin/dashboard_controller.rb

@@ -28,6 +28,7 @@ module Admin
       @pam_enabled           = ENV['PAM_ENABLED'] == 'true'
       @hidden_service        = ENV['ALLOW_ACCESS_TO_HIDDEN_SERVICE'] == 'true'
       @trending_hashtags     = TrendingTags.get(7)
+      @profile_directory     = Setting.profile_directory
     end
 
     private