[bug 1018727] Backfill translations

This adds a "backfill translations" form to the admin allowing us to
backfill missing translations for responses. It's rough, but probably
good enough for now.
Fix date_end issue
Merge pull request #320 from willkg/1018727-backfillmenow

[bug 1018727] Backfill translations
[bug 1033852] Better handle unsupported src languages

Adds some code to not try known unsupported languages with the Gengo
machine translator. The human translator has a way to find out what
language pairs are supported--the machine translator does not. Thus we
hard-code the list of known unsupported languages so we don't keep
trying and kicking up errors.
Add list of unsupported languages to admin
[bug 1014874] Fix translate ux

This uses the Django messages framework to return a success message to
the user after clicking a translate button which at least tells you that
you successfully pressed the button.
[bug 1038774] Hide responses from hidden products

* hide responses from hidden products on the dashboard
* hide responses from hidden products in the response view
Fix a goof in the last commit

"Unknown" isn't a product and thus isn't in the visible_products list
unless we explicitly add it.
[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 methods:
* create_test_data
* _test