Update GitHub issue template with CodeSandbox

I linked to our own examples as templates.

Aside: JSFiddle has fatal errors for me on Chrome on macOS, so I couldn't run demos from issues at all.

Author: silvenon

.github/ISSUE_TEMPLATE.md

@@ -2,7 +2,7 @@
 
 **What is the current behavior?**
 
-**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/m4mb9Lg3/4/).**
+**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via [CodeSandbox](https://codesandbox.io/) or similar (templates: [`Transition`](https://codesandbox.io/s/741op4mmj0)|[`CSSTransition`](https://codesandbox.io/s/m77l2vp00x)|[`TransitionGroup`](https://codesandbox.io/s/00rqyo26kn)).**
 
 **What is the expected behavior?**