Red crosses implied that it was bad/unexpected that certain features
were not enabled. In reality, they are options, so showing a green
or grey power-off icon is more appropriate.

Add status of timeline preview as well

Fix sample accounts changing too frequently due to wrong query

Sample accounts are intended to be sorted by popularity

* 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