Implements a new thread rendering pipeline which all posts and the
main thread body are first converted to a `RenderablePost` structure.
During the conversion to this structure, the post body is rendered as
Markdown and the author's email address is converted into the format
required by Gravatar.
Sets up the structure required to carry Tera templates in the
actix-web state.
The (still very barebones) index renderer has been updated to render a
Tera template.