Add new PR template, with reminder to reference an issue

Description of the need

I created two PRs a few months ago, and was waiting for them to get attention. It wasn't until yesterday I by chance saw this, under https://github.com/backdrop/backdrop#developers:

When submitting a pull request, please make sure there is an open issue in the issue tracker and reference it in the description on the pull request.

Proposed solution

Emphasize that PRs need a corresponding issue. Perhaps for PRs, a pull_request.md template could be made, with something like this?

## Description of the Pull Request

A short description of what the PR solves.

## Link to the issue

When submitting a pull request, please make sure there is an open issue in the issue tracker and reference it in this format (replace ISSUE_NUMBER):

Fixes: https://github.com/backdrop/backdrop-issues/issues/ISSUE_NUMBER

As a follow up task, maybe existing PRs could be scanned for missing issue references? It would be sad if too many first time contributors have PRs gathering dust for months or even years ...

Remaining Tasks

• [ ] Create a PR template
• [ ] Find existing PRs without a reference issue and add a comment to alert the creator of this requirement