make CLA requirement a checkbox [ci skip]

.github/PULL_REQUEST_TEMPLATE.md

-Before we can merge your pull request, you'll need to sign our [Contributor License Agreement](https://docs.google.com/a/metabase.com/forms/d/1oV38o7b9ONFSwuzwmERRMi9SYrhYeOrkbmNaq9pOJ_E/viewform)
-(unless it's a tiny documentation change). Fill this out when you get a chance, and let us know in the PR! :100:
+
+
+###### TODO 
+-  [ ] Sign the [Contributor License Agreement](https://docs.google.com/a/metabase.com/forms/d/1oV38o7b9ONFSwuzwmERRMi9SYrhYeOrkbmNaq9pOJ_E/viewform)
+(unless it's a tiny documentation change).