Bug: direct-upload uses "identifier" instead of "identity" in its API

Description

Submissions can have identities, formatted strings that link to the original submitter. This is helpful because otherwise AuTA does not know who made the submission if the submission came from a third party service.

The direct-upload endpoint only looks for an identifier field, which is against both the API documentation (though we should double check this) and the naming in the rest of the application. This causes identity information to be lost.

Priority

Medium - identity information is now lost, but this information is currently not used

Definition of done

When direct-upload accepts identity information via the identity field. The identifier field should be kept for compatibility, for now.