mod_admin_identity: prevent Safari autofilling username/passwords in new user-account forms. Fixes #811

(cherry picked from commit d4e5ac916917f4bea53f623378b04da20c3d2aac)

Conflicts:
	modules/mod_admin_identity/templates/_action_dialog_set_username_password.tpl

(cherry picked from commit 0dcf38733cc801bba88e55d28bd5be8aea11b6f6)