Use CommonMark block markdown style for Kramdown markdown=1

[harding]

Part of #1551

Kramdown allows an HTML block or span element to contain markdown=1, markdown=span, or markdown=block to trigger markdown parsing within that element. Hugo's Goldmark Commonmark parser requires an empty space after the HTML element open and before the element close to trigger the same behavior.

This commit adds the Commonmark behavior in addition to the Kramdown behavior. We'll have to maintain both until we're fully transitioned.

No test for this is provided.

This PR can be partly reviewed by diffing the site build before and after this commit:

git checkout #this branch
make clean build
cp -a _site _newsite
git reset --hard HEAD^
make clean build
cp -a _site _oldsite
diff --ignore-blank-lines -ruNw _oldsite/ _site/ | colordiff | less -R

That will show you the changes. I've spot checked most of the source differences and they look the same on the rendered page on my browser.

[bitschmidty]

Concept ACK, but also want to run locally

[bitschmidty]

tACK

• ran modified version of @harding commands to diff html output
• manually reviewed a few html differences
• visually compared a few page types where there were differences (a topic page, en news, zh news)
• searched for any potentially missed markdown="1" instances in the code

@harding feel free to merge when you are ready

[harding]

Rebased on latest master in combination with #1757, ran all tests, and pushed to master.