Fix for #1671:

1. Avoid circular dependencies when using post-list in a post
2. Make post.text() work anytime. It goes a bit against design principles, but I feel it's worth it because it makes the Post object more predictable.
3. This adds a "magic dependency marker" that rst directives can use which marks the document as basically unstable, making it never be uptodate. That means it will always be rebuilt.

As a refinement, this could be improved so the rebuild is only triggered, in this case, when the timeline changes.