Disable the text extraction feature.

Since we removed the initialiser override from our deployment code which
disables this feature in production, this will default to enabling the
feature, which may start sending extracted attachment bodies to
Rummager. Since this feature was never completed, we've not fully tested
it, and we're planning to remove it in its current form, we shouldn't
inadvertently enable it.

There's one test that assumes text extraction is enabled in the app;
this now stubs out the feature flag.
Merge pull request #1709 from alphagov/disable-text-extraction

Disable the text extraction feature
Update API adapters.

So that we get partial matching for panopticon request body in the test
stubs.
Send updates to search after changing the topics

Also update tests to check that the correct topics are being sent to
panopticon after they're changed.
Move topic retagger test to integration directory

It's not a unit test, so it shouldn't live under test/unit
Merge pull request #1711 from alphagov/register-topic-changes-with-search

Register topic changes with search