Introduce BP_Invitation and BP_Invitation_Manager.
Add BP_Invitation, a new class describing generic invitation objects with methods for adding, updating and deleting invitations and membership requests.
Add a creation routine for the new table that will contain invitations data.
Add BP_Invitation_Manager, a new class that offers helper functions for managing BP_Invitation objects. For most use cases, this class will be extended for new invitation types, like group invitations.