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/models/form/admin_settings.rb

@@ -44,6 +44,8 @@ class Form::AdminSettings
     :preview_sensitive_media=,
     :custom_css,
     :custom_css=,
+    :profile_directory,
+    :profile_directory=,
     to: Setting
   )
 end