New text for SSP question 5

Clearer question to reduce confusion over whether this is the last day
ill or the first day back.

https://www.pivotaltracker.com/story/show/56625890
Apply question 5 text change to v2
Refactor - clearer variable names
Fix off-by-one error on SSP calculator
Apply SSP off-by-one fix to v2
Update test to catch off-by-one
Copy change into v2
Merge pull request #533 from alphagov/56625890-ssp-minimum-days

56625890 ssp minimum days off-by-one fix + copytext