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/admin/dashboard/index.html.haml

@@ -57,6 +57,12 @@
             %span.pull-right.positive-hint= fa_icon 'check fw'
           - else
             %span.pull-right.negative-hint= fa_icon 'times fw'
+        %li
+          = link_to t('admin.dashboard.feature_profile_directory'), edit_admin_settings_path
+          - if @profile_directory
+            %span.pull-right.positive-hint= fa_icon 'check fw'
+          - else
+            %span.pull-right.negative-hint= fa_icon 'times fw'
         %li
           = link_to t('admin.dashboard.feature_relay'), admin_relays_path
           - if @relay_enabled