Form improvements when creating another's account

Detect a logged-in user creating an account for someone else and change
messages:
* heading is "Enter the new account information below."
* username placeholder is "Enter the username", not "Enter your username"
* email field placeholder is "Enter email address" not "Enter your ..."
* submit button is "Create another account"

Also improve JS when e-mail password is checked to remove the password
fields (otherwise the form still submits them and browser prompts to
remember password).

Bug: 47792
Change-Id: I00e0d21d63bf32ad691ea3a27523298478e2d45c
Merge "Form improvements when creating another's account"