generalize notion of invitation and related data model and REST API

The notion of an "invitation" to join a CO should be generalized to a "petition". The types of petition could be (among others) 'Invitation', 'Application', and 'Conscription'. The work flows for the different types of petitions could be different also. Invitation would still involve an email sent to a prospective new CO member. Application would involve someone petitioning to join the CO with an email notification then going to the CO manager for approval. Conscription would involve a CO admin directly adding a new CO member and then an email notification sent to the new member.