Add "public-project?" checkbox to new-project view

Created by: jbornschein

My impression is that being public or not is such a important choice, that it should be displayed when creating a new project. It does not make the new-project page more cluttered because the warning/explaination at the beginning of the page can now be removed.

The default for that checkbox is determined by the global default setting. If this change is in principle acceptable, I will try to write a proper test; Still need to figure out how to do that though.