To better align with the list of hashtags

* Remove "most popular" tab from profile directory, add responsive design

* Remove unused translations

* 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

Fix #5578