Add flag for overriding the labels when generating dbAuth pages

[cannikin]

Right now the label on the username form fields is always Username and password is always Password. Let's add a command line flag so that you can set that at the time you're generating to save you some time from changing them manually across four pages:

yarn rw g dbAuth --username=Email --password=Secret

Now your form input fields will be labeled "Email" and "Secret" on Login, Signup, ForgotPassword and ResetPassword pages.