Fix sorting on publication date so unpublished content appears newer

Problem/Motivation

Sometimes it is desirable to show unpublished content in lists, so admin users can preview things before publication. However, when the list is sorted on a node's publication date, unpublished nodes all have a publication date of zero and sink to the bottom. A more accurate depiction would be for unpublished nodes to appear at the top of the list, as if their publication date was the current time.

Proposed resolution

• Change the default value for unpublished content from 0 to 2147483647 (the maximum value the database field can hold). That will make views sort properly.
• In addition to the published_at node property, add a published_at_or_now node property that equals the REQUEST_TIME when no publication date has been set.

Remaining tasks

1. Make sure this works.
2. Commit it.

User interface changes

None anticipated.

API changes

When implementing hook_publication_date_alter(), if no publication date has been set then $published_at will equal the defined constant PUBLICATION_DATE_DEFAULT, instead of 0.

Data model changes

None anticipated.