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.