affordability get save and load methods

to deal with serializing to session
add two_applicants to initializer
flatten affordability model initialize method
back buttons no longer use javascript
step 1 does not need interest rate changer
affordability can load from store when it is empty
affordability saves and loads from session
remove unused methods from refactoring
we now persist params to session
remove save code
Merge pull request #166 from moneyadviceservice/back-session

Fix back button