Add mechanism to detect duplicate records

`DuplicateDetector` uses a sliding window to identify duplicates that were created within a certain period of time.
It assumes that data is being fed into the class in chronological order.
Deduplicate anonymous feedback (using a sliding window)
Add rake task for nightly deduping

Anonymous feedback coming through the user satisfaction
and report-a-problem mechanisms is sometimes submitted twice. While
there are browser-side mechanisms to prevent double submissions, they can't
be prevented entirely. This change enables running a nightly job for
deduping feedback for the previous day.
Add rake task for running frequent deduping

There is a nightly task to dedupe anonymous feedback from the previous
day. However, there are still duplicates that show up when feedback
for the current day is viewed (this is especially obvious for user
satisfaction feedback).

This change enables a deduplication task to run every 5 minutes which eliminates
the most recent duplicates.
Add deduplication integration spec
Merge pull request #11 from alphagov/move-deduplication-across-from-support

Move deduplication across from support
Fix `whenever` schedule to repair deployment

This change means that the task runs at 3, 8, 13, etc minutes past
the hour.
Merge pull request #14 from alphagov/fix-schedule-interval

Fix `whenever` schedule to repair deployment