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

config/settings.yml

@@ -15,6 +15,7 @@ defaults: &defaults
   site_contact_username: ''
   site_contact_email: ''
   open_registrations: true
+  profile_directory: true
   closed_registrations_message: ''
   open_deletion: true
   min_invite_role: 'admin'