Move service feedback perisistence to a worker

This allows doing expensive operations (eg organisation lookup
for service feedback) without delaying the form submission on
the frontend, and may result in a slightly quicker response to the frontend app.
Merge pull request #12 from alphagov/offload-service-feedback-persistence-to-queue

Move service feedback perisistence to a worker
Move `whenever` schedule into the app

The schedule isn't secret, nor does it vary across environments, so it should
live together with the app.
Merge pull request #13 from alphagov/move-schedule-to-app

Move `whenever` schedule into the app
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