LP#1851524: fix adding a bucket when saving a new carousel

LP#1851524: fix adding a bucket when saving a new carousel

This patch fixes a regression where saving a new carousel
would not result in its bucket being created due to the
post-save callback never getting invoked.

This patch does not fix any carousels that do not have
buckets due to this bug. Because such carousels will
not function anyway, they should be deleted and
recreated by the local admin.

To test
-------
[1] Create a new carousel in local adminstration. Open it
    in the edit modal again and note that no record bucket
    is linked to it.
[2] Apply the patch and repeat step 1. This time, the
    bucket should be created.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>