Fix shortcode in markdown headers

Fix shortcode in markdown headers

This issue was introduced as a fix to shortcode not working in RST.

One could argue that Blackfriday and friends should handle `#` in titles, but that will be a discussion
for another day.

The new placeholder pattern should be RST safe and work with titles.

And now with a test so this doesn't break again.

Fixes #2192
Fixes #2209
Closes #2210