Because Persona is going to be decommissioned by Mozilla in
November 2016, we want to discourage new sites from using it.
An easy way to do that, is to disable it for newly installed
sites, or when upgrading sites where it is not yet in use.

behatnotneeded: Can't be tested in behat

Change-Id: I7a986d8610a7bed0e7e1968912a242320b8ac44f