redesign to coordinate with other mf2 plugins and themes

Right now, rendering is called directly by the creation process. It should be called by filter. This was originally an option, but was removed in a previous commit.

This code should be less tightly intertwined with the Micropub server code.

Start by tracing where rendering is called from the creation process and how the filter path currently works. Review the previous commit that removed the option and map the Micropub server coupling before proposing a design. Done means rendering is coordinated through the filter and is less tightly intertwined with the Micropub server code.

Written by the indexing model from the issue text.