Add on_behalf_of field to Membership.
Switch to use ForeignKey for Organization parent.
Mandate required fields of Post and Membership.

organization is required on a Post; person and organization are
required on a Membership. Use the post's organization in the
add_role and add_person functions, and remove the properties
for accessing membership_set/post_set, instead using the default
Django way of accessing.
Merge branch 'add-on_behalf_of'
Merge branch 'make-parent-foreign-key'
Merge branch 'mandate_required_fields'