Merge branch 'cs-issue-pr-templates' into 'master'

Add Issue/PR Templates to deter issues/contributions on the GitHub mirror of the project

## What does this MR do?
Adds GitHub-specific `ISSUE_TEMPLATE.md` and `PULL_REQUEST_TEMPLATE.md` files in a `.github` directory. To prevent new issues/PRs, I figured it'd be good to direct users/contributors to open issues/contribute code in the "correct" project.

## Are there points in the code the reviewer needs to double check?
Wording/phrasing, mostly.

## Why was this MR needed?
The GitHub issue tracker is being closed, and PRs on GitHub haven't been accepted for a while now. This was discussed briefly during the GitLab Strategy Session at the Austin Summit.

cc: @dzaporozhets @rymai  @MrChrisW @dblessing @virtuacreative @amara  

See merge request !4324

.github/ISSUE_TEMPLATE.md

+We’re closing our issue tracker on GitHub so we can focus on the GitLab.com project 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 log into GitLab.com using your GitHub account.

.github/PULL_REQUEST_TEMPLATE.md

+Thank you for taking the time to contribute back to GitLab!
+
+Please open a merge request [on GitLab.com](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests), we look forward to reviewing your contribution! You can log into GitLab.com using your GitHub account.