Add optional notification for new users that are in the inactive state.
Add tests for 'new inactive user notification' feature.
Fix AdminNotifier, bitrot after upgrade to Ruby 2.x and Rails 3.x.
Remove reference to non-existant users_url
Refactor AdminNotifier.

The old implementation was elegant, but not super useful in the
situation where you want to send different e-mails from a method.

Due to the way ActiveMailer is implemented, Rails would reuse the
Mail::Message object for sending the messages, which makes this
impossible to test.

So... I threw out the fancy solution in favor of something that also
works and can be tested fully.

I also added a number of tests.
Merge branch 'master' into 2681-new-inactive-user-notification
Change default for

  new_user_notification_recipients
  new_inactive_user_notification_recipients

in config/application.default.yml to an empty array, so that it is
obvious how to specify multiple recipients.
Provide link to users page in notification e-mails.
Switch to using auth helpers instead of messing with Thread.current in the user tests.
Refactor user notification tests
Merge branch 'master' into 2681-new-inactive-user-notification
DTRT with system_user in our user test case. Partial revocation of
commit:e9fc734b
Merge branch 'master' into 2681-new-inactive-user-notification
Merge branch 'master' into 2681-new-inactive-user-notification
Merge branch '2681-new-inactive-user-notification'

refs #2681
2872: Ensure name link tail_uuid == owner_uuid. refs #2872
2872: Change default order of folder contents. refs #2872