Issue template & pull request template [ci skip]

ISSUE_TEMPLATE.md

+When you open an issue, it would be super helpful if you could include the following information: :100:
+- [ ] What browser/OS are you using? (Or are you using the Mac App?)
+- [ ] What version of Metabase are you using?
+
+Thanks! Luv, Team Metabase :heart_eyes_cat:

PULL_REQUEST_TEMPLATE.md

+**TODO** Blurb about Metabase's rigorous design process, etc.
+
+One more thing: 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! :100: