Scheduled posts

Make possible to create posts that are not published immediately, but at some moment in the future.

A possible implementation is to create a separate table containing the posting ID, feed name and the target date/time of the publication. The data will be passed through PostingText.publications structure. The scheduled posts will be available immediately, but the story will be created at the target date/time. Need to ensure that the posting will not be erased as not referenced from any feed (or create a special feed for temporarily keeping them).

In the UI scheduling will be switched on/off with a button near the posting's date. The posting's date will be the target date of the publication. "Change date/time..." in the posting's menu can be used to change the target date/time later. The dialog should also contain a button for immediate publishing.

Since the scheduled posts will not be present in regular feeds, need to find a way to display them in a form of "Scheduled posts" feed. It may be one of "system feeds".

Start by tracing the PostingText.publications structure and the posting date/menu UI described in the issue. Map how posts are retained, published into stories, and exposed through feeds. Done means future publication, date changes, immediate publishing, retention, and a Scheduled posts feed all work together.