Fix file attachment updating.

When I pushed these changes, I had made a couple last-minute changes that
didn't end up completely committed to git, which meant that things were in a
relatively broken state. This change fixes two issues with file attachment
histories.

First, the attribute name when creating new file attachments had changed from
`attachment_history_id` to `attachment_history`. This was reflected in most
places but a couple didn't get the update.

Second, newly-created FileAttachmentHistory objects weren't being properly
associated with the right review request, which meant that future updates would
be rejected.

Testing done:
- Created some new file attachments and then updated them.
- Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/6511/