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/locales/en.yml

@@ -229,6 +229,7 @@ en:
       config: Configuration
       feature_deletions: Account deletions
       feature_invites: Invite links
+      feature_profile_directory: Profile directory
       feature_registrations: Registrations
       feature_relay: Federation relay
       features: Features
@@ -376,6 +377,9 @@ en:
       preview_sensitive_media:
         desc_html: Link previews on other websites will display a thumbnail even if the media is marked as sensitive
         title: Show sensitive media in OpenGraph previews
+      profile_directory:
+        desc_html: Allow users to be discoverable
+        title: Enable profile directory
       registrations:
         closed_message:
           desc_html: Displayed on frontpage when registrations are closed. You can use HTML tags

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'