add embed query to allow expansion of memberships

This adds an embed query parameter which takes the same form as django's
join specifiers and allows the expansion of the person or organisation
sections of the memberships of a person or organisation

e.g adding ?embed=person to the end of a resource URL will expand all
the persons entries in the memberships of a resource.
?embed=person__organization will expand the persons entries and then the
organization entries within those.

This also add a basic query param such that appending ?basic to a
resource will not fill in the memberships. This is to cut down on
document size if membership information isn't required.