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/settings_controller.rb

@@ -26,6 +26,7 @@ module Admin
       show_known_fediverse_at_about_page
       preview_sensitive_media
       custom_css
+      profile_directory
     ).freeze
 
     BOOLEAN_SETTINGS = %w(
@@ -37,6 +38,7 @@ module Admin
       peers_api_enabled
       show_known_fediverse_at_about_page
       preview_sensitive_media
+      profile_directory
     ).freeze
 
     UPLOAD_SETTINGS = %w(