Implement common markdown blocks infrastructure

Takes the infrastructure from !117 (closed) and addresses the issue with recursing into Jinja.

The approach that @gaazzopa took of using Jinja2 environments sparked an interesting idea, and as a result I kept the fact that the common blocks go through a round of being rendered by Jinja (very simplistically, with no variables etc.). In this way we can achieve theming, with the ability to extend existing text, using standard Jinja blocks, rather than macros.