Update account creation form validation

Update account creation form validation

Use the cancreateerror returned from list=users&usprop=cancreate for
username validation.

Use the new action=validatepassword to validate entered passwords.

This also injects the resulting errors in the style of HTMLForm's field
validation rather than at the top of the form.

Change-Id: Ie8c1270eb605367556fe36b0b2080eb3f957dc54