Keep availability 'preview' on unpublished variant

The 'hippo:availability' property of the unpublished variant now is emptied as a document is published. This was used in the past to detect that changes had been made to the unpublished variant after publication.

Since the summary state 'changed' is now derived from modification dates, however, the availabilities of the published and unpublished variants can remain constant.

With this change, the same variant will always be the 'preview'.