change namespace from 'project' to 'projects'
This is required to preserve URLs pointing at things like https://www.ohwr.org/project/fmc-adc-100m14b4cha/wikis/home Lots of existing URLs are like these.
For the time being, there is a redirection /projects/ -> /project/ that fixes this (see https://forums.ohwr.org/t/projects-vs-project-in-url/848034)
Changing the namespace would have the consequence that all the /project/ references in the migrated site (wikis!) would be broken again...