LP 1427392 - Allow deletion of Hard Due Date Values. Also, don't update

LP 1427392 - Allow deletion of Hard Due Date Values. Also, don't update
hard due dates to ceiling dates that have passed.

Fixes two related issues with hard due dates.

- Exposes the selector in the Hard Due Date Values interface so past
  entries that are no longer valid can be deleted via the client.
- Prevents the current ceiling date from being updated to a past date
  based on an old hard due date value.

Testing plan:

To reproduce the issue:

- Create a Hard Due Date with a Current Ceiling Date in the future
- Create an associated Hard Due Date Value with an active date earlier
  than the current date and a ceiling date earlier than the current date.
- Run the hard due date updater

Note that:

- The Current Ceiling Date is updated to an earlier date based on
  the Hard Due Date Value entry.
- It is not possible to select and delete the Hard Due Date Value
  entry.

Apply the patch and retest

Note that:

- The Current Ceiling Date has not been updated to the past date.
- Past Hard Due Date Values can be selected and deleted.

Signed-off-by: Michele Morgan <mmorgan@noblenet.org>