3411: set default collection expiration policy

* Added default_scope to Collections restricting them to collections that
  have not yet expired

* Added a default_trash_lifetime setting in application.default.yml,
  exposed in the discovery doc setting 'defaultTrashLifetime' (default:
  2 weeks)

* Added test fixtures to exercise fetching expired collections and
  collections that expire in the future.
3411: code review

Feedback from code review:

* Dropped unnecessary lambda from Collection.default_scope.
* Set owner_uuid of new collection test fixtures to user 'active',
  removed unnecessary permission link test fixtures.
* Removed permit_unsigned_manifests from unit tests
* Added tests for :show and :update to expired and unexpired
  collections.
* Added rake config:check test to make sure default_trash_lifetime is at
  least 24 hours.
3411: code review feedback

* Ensure each functional test issues only one request to the controller
* $application_config['default_trash_lifetime'] -> Rails.configuration.default_trash_lifetime
Merge branch '3411-expire-collections'

Closes #3411.