Editing a Deadline Fails Because Deadline ID Is Not Submitted

Issue Description:
When a user edits an existing deadline description and saves the changes, the application throws a DeadlineDoesNotExist error.

Expected Behavior:
When a user edits and saves a deadline description, the correct deadline should be submitted, the existing deadline should be found successfully, and the updated description should be saved without throwing a DeadlineDoesNotExist error.

UI:

Start from the deadline editing UI and trace how the edited deadline is submitted when its description is saved. Verify that the existing deadline ID is included, then confirm that the correct deadline is found and its updated description saves without a DeadlineDoesNotExist error.

Written by the indexing model from the issue text.