There were a few long-term inconsistencies between edit mode and render

mode for Markdown code blocks:


Whitespace in a code literal wasn't preserved in render mode, even

   if present in edit mode.


The start and end of a fenced code block in edit mode was getting the

   background color of a code literal, which looked broken.


Some colors were different for code inside a code block between

   rendered mode and edit mode.


This change fixes these three issues, making code look a lot more

consistent between the two modes.

Tested a handful of code blocks and inline code, making sure presentation

was mostly consistent where possible.