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/views/layouts/public.html.haml

@@ -9,7 +9,8 @@
           = link_to root_url, class: 'brand' do
             = image_tag asset_pack_path('logo_full.svg'), alt: 'Mastodon'
 
-          = link_to t('directories.directory'), explore_path, class: 'nav-link'
+          - if Setting.profile_directory
+            = link_to t('directories.directory'), explore_path, class: 'nav-link'
           = link_to t('about.about_this'), about_more_path, class: 'nav-link'
           = link_to t('about.apps'), 'https://joinmastodon.org/apps', class: 'nav-link'
         .nav-center