Enable Devise to be paranoid

Why:

* leaking existing emails if not necessary is a bummer -
  eliotsykes/rails-security-checklist#21

This change addresses the need by:

* turning on Devise's paranoid mode and sending an email when a password
  is changed.

https://trello.com/c/EOF91FHv/338-make-census-send-password-was-changed-emails

config/initializers/devise.rb

@@ -75,7 +75,7 @@
   # It will change confirmation, password recovery and other workflows
   # to behave the same regardless if the e-mail provided was right or wrong.
   # Does not affect registerable.
-  # config.paranoid = true
+  config.paranoid = true
 
   # By default Devise will store the user in session. You can skip storage for
   # particular strategies by setting this option.
@@ -111,7 +111,7 @@
   # config.pepper = '44b952d1823bb91660b12456eeffe18f0c5ff5143cbee7f635b1f62ba03efeedfd1daa975f16da03e2837a1494e4915966ab3eeaa1473e47e89d55070c799e3a'
 
   # Send a notification email when the user's password is changed
-  # config.send_password_change_notification = false
+  config.send_password_change_notification = true
 
   # ==> Configuration for :confirmable
   # A period that the user is allowed to access the website even without