Add ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md to point contributors...

Add ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md to point contributors toward the GitLab.com repository.

This adds templates for Issues/Pull Requests in a `.github` directory. These only effect issues/PRs opened in the GitHub mirror of the GitLab project. As we're shutting these down, I thought it'd be good to direct users/contributors to open issues/contribute code in the "correct" project.

.github/ISSUE_TEMPLATE.md

+We’re closing our issue tracker on GitHub so we can focus on the GitLab.com issue tracker and respond to issues more quickly.
+
+We encourage you to open an issue on the [GitLab.com issue tracker](https://gitlab.com/gitlab-org/gitlab-ce/issues). You can login on GitLab using your GitHub account if you'd like to contribute an issue.

.github/PULL_REQUEST_TEMPLATE.md

+Thank you for taking the time to contribute back to GitLab! Due to the high number of contributions we get, we’re unable to review requests on GitHub right now.
+
+We encourage you to open a merge request [on GitLab.com](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests). You can login on GitLab.com using your GitHub account.