Build: Advise to create test cases on JS Bin or CodePen, drop JSFiddle

JSFiddle doesn't support IE (even 11) anymore so we shouldn't advise users
to use it to create test cases. To make people have a choice, add CodePen
to the list.

Also, link to specific starter templates so that novices don't need to spend
time thinking how to set up the basic structure.

Closes jquerygh-4289

Author: mgol

.github/ISSUE_TEMPLATE.md

@@ -11,7 +11,7 @@ Bug Reports:
   *  Which browsers are affected?
   Provide a *minimal* test case, see https://webkit.org/test-case-reduction/
   Use the latest shipping version of jQuery in your test case!
-  We prefer test cases on https://jsbin.com or https://jsfiddle.net
+  We prefer test cases on JS Bin (https://jsbin.com/qawicop/edit?html,css,js,output) or CodePen (https://codepen.io/mgol/pen/wNWJbZ)
 
 Frequently Reported Issues:
   * Selectors with '#' break: See https://github.com/jquery/jquery/issues/2824