Use mgnl:folder instead of mgnl:content for the /public node in users workspace

PUR module is using mgnl:content nodetype for the /public node where it stores the publicly registered users. But the nodes for the core users (/system and /admin) are of the mgnl:folder type. This causes problems in the Security App for M5.

The fix should change the install tasks to use mgnl:folder instead of mgnl:content, and update tasks to change the primary node type of the existing /public node.