This aims to give a permanent solution to #457 and similar.
Since upstream continuously add new ways to reach the "New project" pages, it makes sense to have them display a very clear message instead of buggy forms.
The message can be customized in the admin area (Settings > Account and Limit), so that instance-specific policies and hints can be stated.
Even when we manage to hide all links to the New Project form to users without creation rights, this is often so surprising to them that they believe there is a bug. So perhaps we should simply reinstate all these links and let the explicit message do the job.