allow full rendering of subboard-details

This is one thing that annoyed me from the first release... subboards on the boards-overview aren't rendered with their full details, only a to-be-generated url and their name, preventing a more detailled usage.

This pull request allows the full and complete rendering of subboards (including the full url without using a workaround with a snippet) using the same placeholders like in the other chunks. The subboards are cached like the parent boards in the same script for performance-reasons.