This addresses an issue reported on the Forum where setting a password for
an Agent upon creation doesn't save the password in the backend. This means
if you set a password, click Create, and then try to login as the agent you
will be denied access (as there is no password saved). We store the
PasswordResetForm data in the session for later use but we do not actually
use the form data anywhere in the `Staff::update()` method. This updates the
`Staff::update()` method to include a check for the form data and if exists
sets the password and saves it in the backend. This maintains current
functionality where if `Send the agent a password reset email` is enabled
then we do not set the password, instead we send the Agent a registration
email.

In addition, this updates the `StaffAjaxAPI::setPassword()` to include
password validation so if a password is not valid we do not save it.
Instead, we will keep the modal open and throw validation errors until a
valid password is given. This means the form data will not be stored in the
session until we have a valid password.