lib/util.js: Modernize Javascript code and usages

[StephenChan]

• Use ES6 classes and syntax.
• util.trimSpaces() is obsoleted by the standard String.prototype.trim().
• Function.prototype.curry() is probably not necessary. Better to learn and directly use the Javascript idiom(s) for partial function application. Not sure off the top of my head, but may involve bind() or apply().
• String.prototype.endsWith() and String.prototype.startsWith() are already built into modern browsers.
• String.prototype.format() is obsoleted by template literals.
• $(document).ajaxSend() should either be canned if not needed, or reworked to work for fetch() as well as jQuery.ajax(). The current Django docs show how to rework this in a way that's agnostic to the Ajax implementation.
• Try not to rely on jQuery in general.
• While we're at it, update docs/design/browser_compatibility.rst to stop caring about IE compatibility altogether; doesn't seem worth the trouble for us.