Allow for site-wide issues email address default value

It's currently not possible to automatically add a mail_copy e-mail address to new projects upon project creation. This is by design, the comments in project_issue_project_insert() say it all:

  // Setup reasonable defaults: enable issue tracker and standard
  // components. If maintainers want to change these, (or any of the
  // other issue-related project settings, like the email notifications)
  // they'd use the "issues" subtab of the project edit tab (since none
  // of these fields are present on the initial project add form).

and the insert query cannot include any mail_copy value.