Add "Fixes: number" to PR template for auto-closing issues

[khos2ow]

Description

Added GitHub issue section to PULL_REQUEST_TEMPLATE.md with Fixes: #<issue_id> format. With this addition, the referenced issue will automatically be closed when PR gets merged.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Screenshots (if appropriate):

How Has This Been Tested?

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
  Testing
• I have added tests to cover my changes.
• All relevant new and existing integration tests have passed.
• A full integration testsuite with all test that can run on my environment has passed.

[yadvr]

This can be part of the description? The pull request form is already very long.

[khos2ow]

That's true, it can (and should) even be in the commit description as well. I only added another section since we already have some other sections for different purposes, and make it more unified (in terms of placement of Fixes: #<id>)

[rafaelweingartner]

Can I suggest one other thing? What about changing ## GitHub Issue to ## GitHub Issues/PRs
Then, you could use:

<!-- List the PRs and issues that this PR addresses. E.g. "Fixes #.., #.., and so on "-->
<!-- Fixes: # -->

A PR can also be used to close (address) other PRs' topic and issues at the same time.

[khos2ow]

That's true! Although the correct syntax would be Fixes #..., Fixes #..., .... The keyword needs to be specified before number everytime (reference).