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 pull request #72 from oerpub/semantic-block-type-indicator

Show class name when hoveringover block of unknown type
Merge pull request #71 from oerpub/no-copy-on-solutionblock

Consult with the plugin as to what buttons to place
Merge pull request #69 from oerpub/56802896

fix sortable timing bug regarding editable activation
Merge pull request #70 from oerpub/56713414

prevent latent class='' from being left on table cells and rows
Merge remote-tracking branch 'origin/master' into math-selected-fixes
Merge branch 'math-selected-fixes' into testing