Implements cross-board reference syntax in formatters.

The syntax is >>>/board/topic/anchor (e.g. >>>/demo/123/10-) with
topic and anchor omittable. This allow usage such as >>>/foo for
pointing user to other board, and such.
Fix a bug where cross-topic link do not link to recent.
Update JS components to preview cross-reference links.
Merge branch 'feature/cross-reference' into develop