[bug 947767] Nix authentication for API stuff

We don't do any authentication-requiring things with our API, so it's
best to nix the authenticators for now. The authenticators were causing
logged in users to get an HTTP 403 when submitting a Firefox OS feedback
form because of csrf token related things.
Fix response view re: non-existent products

The product field in the feedback_response table is not a ForeignKey to
the feedback_product table. The checking code didn't account for that
and died when the product didn't exist. This fixes that.
[Bug 1030905] fjord feedback api tests for dates

Test the get api when filtering by date.
Add the following tests:
* test_date_start
* test_date_end
* test_date_delta_with_date_end
* test_date_delta_with_date_start
* test_date_delta_with_date_end_and_date_start
* test_date_delta_with_no_constraints
And the following helper method:
* create_test_data
* _test