Adds editor interface for membership roles within N2 console.

N2 already has great support for rights management in the content tree, allowing read/write/publish/administer rights to be granted on a role level for branches in the tree. There are currently a fixed number of roles, defined by N2 itself.

With this commit, new functionality is added to N2, allowing for roles to be created and deleted from the management interface. For example, this could be used to create a role for editors of a specific branch (department/language/country) of a site.