Disable spell check in math and code blocks #6

Any block type where spellcheck doesn't make sense, like math and code, we maybe could add spellcheck="false" in the editor.

And maybe also: autocapitalize="none" autocorrect="off" autocomplete="off" (gotta look into it more, I think these may not be as commonly supported but prob couldn't hurt?)

orig at GitHub: https://github.com/hyperlink-academy/leaflet/issues/187