WIP: implement job to discard User records created by people signing up but never confirming their email address.

The interval after which unconfirmed signups are considered for being discarded is configurable in application.rb, in the config.discard_unconfirmed_signups_after variable.

Not yet implemented: unit tests, static schedule for the job.
Added unit test.
Refined unit tests.
Added unit tests for CleanupSignupsJob.
Added index to users table on confirmed_at, confirmation_sent_at columns.

This will speed up searching for old unconfirmed signups to destroy in CleanupSignupsJob.
Added daily static schedule for CleanupSignupsJob.