FIX: honor `email_token_grace_period_hours` setting correctly
FIX: honor `email_token_grace_period_hours` setting correctly

The documentation said that the unit for this setting is in *hours*, but the
code is treating it as *seconds*. Also, `Numeric#ago` has been deprecated since
Rails 4.1 (precisely to help prevent these kind of bugs) and will go away in
Rails 4.2.

natefinch/discourse,natefinch/discourse