This makes a few changes to the new inline code and code block styles
for the Markdown WYSIWYG stylesheets to make the text easier to read and
to help keep the edit and render content lined up, as we had it before.

The spacing around the inline code blocks have been reduced, and the red
and border colors changed to increase contrast. This helps keep the text
readable while also preventing the inline blocks from bumping up against
neighboring rows. There's also some padding that helps keep the rendered
inline code box the same width of the source text (backticks included),
so that those editing content will have a better sense of line width,
preventing them from going over column limits.

The code blocks have had their padding and alignment updated to keep the
rendered code aligned with the editted version. The top and bottom
borders are placed where the opening and closing fenced code block
markers (the triple backticks) would be.

With these changes, the users will now have a much nicer experience
editing the Markdown, getting the benefits of the new code styling with
better readability and improved consistency between source text and
rendered results.

Tested on Chrome and Firefox. Flipped back and forth between edit and

render modes.
Tested the review request fields, review dialog, and comment replies.

The latter two don't have left/right borders, but it's not bad looking

without them, and can be addressed on the Review Board side.