Permit :id for nested attribute fatality notices

Following-up on:
https://github.com/alphagov/whitehall/pull/1862

I missed adding this important bit of the fix,
which permits the `:id` of the destroyed nested
attribute, allowing rails to retreive the correct
associated record and destroy it.

The earlier PR already contains an integration
test to test this behavior.
Merge pull request #1883 from alphagov/fix-fatality-fields-duplication

Permit :id for nested attribute fatality notices