Use resource params for state transitions

Moves params used for control flow between form states into the resource params,
e.g. params[:was_employed] becomes params[:respondent][:was_employed] with this
commit. This commit also stores these things in the DB because the user can go
back and forth arbitrarily between states, we need to know their value if a
user revisits a particular state so we can show the page how they left it. In
future we can probably be smarter about these attributes by computing them from
other attributes but this is fine for now.