Adding a project link without a sort order hangs the application

[redrun45]

When adding custom links to a project (application/controllers/private/Administer_projects.php -> update_add_project_url()), if order is omitted, the server returns error 500. The client-side JS, expecting an AJAX response that never comes, displays a loading-spinner indefinitely.

While better error handling would be nice, the most direct fix would likely be to add a default value to project_urls.order. Since it's used for a relative sort, the value does not need to be unique, even among "sibling" links. Defaulting to 1 might be all we need for this particular issue.