prefill ticket form from the model - fixes #5657

email and regarding user fields can be set to defaults based on
created_by user.

If these fields are emptied by the submitting user they will be set to  whereas they are nil if they have not been initialized. In that case we will use meaningful defaults from the user who created the ticket.
destinguish user.email from user.email_address

use the former if you want a working email account or nil, the
latter if you want the email address associated with a given
user no matter if the user actually has an email account or not.