This adds a class `rendered_markdown` for all the elements which have
rendered markdown content; This is done to add different styles for
rendered content in day mode and night mode.
Also replace the element selectors from CSS to use the class.
This should make it easier to find the templates that are actually
part of the core webapp, instead of having them all mixed together
with the portico pages.
Vaibhav
Rohitt Vashishtha
Steve Howell
Anders Kaseorg
Tim Abbott