Misc: Add issue and pull request templates

Fixes jquerygh-2929
Closes jquerygh-3070

Author: dmethvin

.github/ISSUE_TEMPLATE.md

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+<!--
+Feature Requests:
+  Please read https://github.com/jquery/jquery/wiki/Adding-new-features
+  Most features should start as plugins outside of jQuery.
+
+Bug Reports:
+  Note that we only can fix bugs in the latest (1.x, 2.x, 3.x) versions of jQuery.
+  Briefly describe the issue you've encountered
+  *  What do you expect to happen?
+  *  What acually happens?
+  *  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 http://jsbin.com or http://jsfiddle.net
+-->
+
+** Description**
+
+
+** Link to test case **
+

.github/PULL_REQUEST_TEMPLATE.md

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+### Summary ###
+<!--
+Describe what this PR does. All but trivial changes (e.g. typos)
+should start with an issue. Mention the issue number here.
+-->
+
+
+### Checklist ###
+Mark an `[x]` for completed items, if you're not sure leave them unchecked and we can assist.
+
+* [ ] All authors have signed the CLA at https://contribute.jquery.com/CLA/
+* [ ] New tests have been added to show the fix or feature works
+* [ ] Grunt build and unit tests pass locally with these changes
+* [ ] If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com
+
+Thanks! Bots and humans will be around shortly to check it out.