Remove use of keyword "delete" in forum coffee code to fix IE8 bug LMS-441.

Change client-side code references from "delete" to "_delete". Some of these
were method names (or would generate method names), which breaks in IE8
because "delete" is a JavaScript reserved keyword. foo["delete"] is ok, but
foo.delete is not. This also makes forums marginally functional in IE8,
though the left-nav still has some display issues.
notify students for NotFoundErrors from capa_module, improve tracking