Refactor nightly deduplication spec

This allows for test data reuse.
Bugfix: deduplication of recent feedback actually works

This change also:
- adds an integration-level spec which would have caught the bug
- removes the unit-level spec for `DeduplicationWorker`.

The unit-level spec is redundant because the logic is covered by the integration-level
spec. Furthermore, using mocking meant that the error slipped through, making the spec
somewhat pointless.
Deduplication bugfix: add all anonymous feedback types

There are several types of anonymous feedback (problem reports,
service feedback, long-form contacts) and they are all persisted in the
`anonymous_contacts` table using STI.

The deduplication process only references the `AnonymousContact` superclass,
but in order to mark a record as a duplicate, Rails needs to instanciate it
and if the class specified in the `type` column of the record isn't found
within the app, an exception is thrown.

This means that for de-duplication to actually work properly, the classes
for all types of feedback need to be present and loaded.