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.