Use separate form elements for each section
Extract the download button to a partial
Extract main form nav to a partial
Extract partial for form sections
Dry up the nav partial
Add Residence model for past residences

This is a big reorganization for addresses person relationships.

- Kinds of people (household members, landlords) and kinds of
  addresses (residential, mailing) are distinguished by join tables,
  instead of the less flexible single-table inheritance method.

- The Residence model groups together an address, a landlord, a time period, and
  a reason for leaving.  This is more logically consistent than having addresses
  belong_to people and landlords belong_to applicants.
Create household_members and mail tables
Remove the MailAddress class
Make Address, Person, Residence depend on Applicant
Fix applicant associations for new model
Person belongs_to :mail_address
Use belongs_to for applicant identities
:destroy records dependent on Applicant

Address isn't consistently owned by anything, so it doesn't get destroyed along
with the owning Applicant.  Need to fix that.
The main form renders once again
Make mail address editable again
Make the form mostly usable again
Seed other residence fields
Fix field filling for residence addresses
Factor out the main_form/section partial, disable ajax updates
Quiet the dev logs