Improve table styling in rendered Markdown

Review Request #8970 — Created May 25, 2017 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x
4e1d3c5...

Reviewers

This patch adds some better styling to our rendered Markdown. We now
render vertical borders and emphasize the border that separates headers
from content.

Tested this with /r/8969/