Disable autocompletion in password fields by default

[dlh01]

See https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion.

We had a head-scratcher on one site recently where a password field on a submenu page kept being changed because a user's in-browser password manager kept autofilling it whenever the submenu was opened. Is there ever a case where that behavior would be desirable by default?

[nikolareljin]

@dlh01 We have switched to store the passwords into plaintext since the problem definitely prevents most of the password tools from using the desired FieldManager's field.
One possible way to change this:
#755