Throttle (debounce) consecutive clicks on run button [fixes #66270574, #71766756]

In IE 11, clicking the Run button would call onAttempt twice. This can be tested by setting a breakpoint on onAttempt() in the IE dev tools on BrowserStack.

This fix adds the underscore debounce utility to our utils class and debounces the run button click callback so that it does not get hit more than once in 250ms.

With this fix in place, the breakpoint only gets hit once ([test vid](http://screencast.com/t/2WDXsIT2wrSv)).