If there's only 1 project, don't force user to pick at node/add/[issue-type]

This is sort of the other side of the problem from #1822482: Deal with 'create an issue' links for sites with more than 1 issue node type.

In D6, our magic /node/add/project-issue landing page is smart enough to know that if there's only 1 project the user has access to, we just directly redirect there:

  // See if there's only one project the user has access to, and if so,
  // redirect directly to the issue form for that project.
  if (count($short_names) == 1) {
    drupal_goto('node/add/project-issue/' . key($short_names));
  }

That's not in the D7 version, and it's making life a little complicated for the single-project usecase: #1980370: Provide use-case for single projects

This doesn't effect d.o, so I'm not tagging it for the d.o upgrade, but it'd still be a nice touch, especially in light of #1952792: Make the default project + project_issue install easy to use for non-software project management