EmploymentForm fixes

  - Do not save employment if user set #was_employed to no
  - Destroy existing employment if user set #was_employed to no
  - Pre-select no the first time user sees page (instead of yes)
Merge pull request #269 from ministryofjustice/bug-fix/preselect-no-do-not-save-employment-if-was-not-employed

EmploymentForm fixes