Bug 1422492: Make user enter current password if changing their username

As the change username option only appears on account index page for
those users that have an auth method allowing it we should make the
user also supply their current password when changign username

We hide the confirm password box for username change until they try to
change it

behatnotneeded

Change-Id: Ic05d3c258d331305ae5c07b952ea2a561a8badf1
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit 8ad4343d)