Split the Resident model into Applicant and Person

Housing application forms require information about many different people
associated with a single applicant.  Family members, cohabitants, employers,
landlords, and so forth.

To avoid duplication of many different fields related to the identity of a
single person, Person is its own model.  There's a new model, Applicant, which
represents the person applying for housing.

Applicant has a "self" attribute, referring to the applicant's identity.  It
will require the addition of more person-identifying foreign keys, for family
members and other people involved with the applicant.