Add the ability to make certain panes read-only.

[lisunshiny]

In our classroom, we did a Do Now this morning that involved our students changing the text of a div tag in JavaScript from "something" to "something else". They, being the growth hackers that they are, proceeded to change the text in html 🤦🏻‍♀️ instead of writing $("div").text("something else");. It'd be useful to add the ability to lock editing in certain panels. Looks like this is possible in Ace with editor.setReadOnly(true) (ajaxorg/ace#406).