Allow for configuring the preview expiry time default value

It made more sense for this to be configurable via the core Drupal\Core\Site\Settings instead of actual configuration.

I would prefer to have it in configuration though. settings.php can override config :)

Moved to config :)

Addressed the comments. This is ready for another review.

Merged into 2.0.x and cherry-picked to 1.0.x, thanks!

Automatically closed - issue fixed for 2 weeks with no activity.