Add cache-busting to request response notification

Users get a new response email, click the link, but get a cached page.
This is a quick fix to ensure they always see the response when clicking
the link.
Remove duplication from new correspondence urls
Move date helpers to DateTimeHelper
Add specs for DateTimeHelper#year_from_date
Add specs for DateTimeHelper#simple_time
Minor tidying of DateTimeHelper and specs
Remove DateTimeHelper#year_from_date

DateTime, Date and Time all have #year methods
Merge branch 'issues/788-notification-email-cache-buster' into rails-3-develop