Only run mathjax once for root editor.

I selectively recovered this from mathjax-streamline. Should help
performance. I have no idea if it will break anything.
Merge remote-tracking branch 'origin/master' into math-selected-fixes
Merge remote-tracking branch 'origin/master' into math-selected-fixes
Merge pull request #74 from oerpub/math-selected-fixes

Only run mathjax once for root editor.