Issue: Schedule content/blog-post

Implemented in 4.2.x, generally for most content types:
- Schedule future pages, feeds, synonyms.
- Schedule future discussions, and replies.
- Schedule future edits and file updates.
- Schedule future feed entries. Unless the shown content is also scheduled, it might be visible in a different context (like someone else's feed who didn't schedule it, or by reading directly).

Currently you cannot schedule things like adding a chapter to a book, although it is theoretically not too difficult.
We already had to support scheduling adding categories to a page if they are added via the editor when scheduling an edit (mostly due to categories added via BB-Code).
So if there is enough demand we may add this for other kinds of adds.

You cannot schedule changing a page behavior (toggling Book, Anonymous, etc). That is immediate.

Scheduling of edits adds complications:
- Since moderating doesn't freeze pages anymore, neither will scheduled edits. In the case where a page already has scheduled edits, the user will only be able to edit the page if they can schedule edits too. In case they want to make an edit earlier, the editor warns the user that edits will eventually be overwritten if they aren't scheduled late enough.
- Since renaming a page is a potential edit, potentially the renaming can be scheduled. Since many edits can be scheduled for a single page, we keep track of ranges of time when title combinations will become available. The idea is to support scheduling renames across multiple pages, if the pages have each other's names.