Add support for family packs using invitations.
This commit adds support for having family-packs of 5 members including the administrator.
The administrator will be able to create up to 4 invitations links that will be used to register as member of the group.
The new members will enjoy same services as the owner of the group. Services are activated as soon as they register.
New config parameter WOO_CART_GROUP needs to be added into config.ini file.
Also a new add_to_cart URL needs to be configured in WC.