Suppress equation editor

[mackensen]

The equation editor will never render correctly in an email, so we should consider removing the option altogether. Per MDL-52996 and Editor API this should be possible.

[andyzito]

At the moment this is blocked by https://tracker.moodle.org/browse/MDL-64512

[kwiliarty]

I just encountered something today that might be related. Using current LAE 3.8: Sending a youtube link through clampmail gives a bad result in HTML mail, I believe because the Multimedia plugins filter and hitting the link and trying to build a player around it. Turning off the filter allows the email to work correctly. I wonder: should we be preventing filters from applying to the contents of an email? I realize this might be a separate issue.