Proposal: Feature to check that a PR description includes closure keywords

## Expected Behaviour
Addition of a new feature to check the contents of a PR description for a referenced issue number. See: https://help.github.com/articles/closing-issues-using-keywords/ for a full explanation.

## Current Behaviour
A PR is raised with an issue reference but without a closure keyword. This results in manual activity to keep repo issues in sync with project state.

## Possible Solution
Similar to the sign-off check. Test the incoming detail for GitHub closure keywords:

```
close
closes
closed
fix
fixes
fixed
resolve
resolves
resolved
```
alongside an issue reference. Further detail on the format of these is available at the aforementioned link.

On receipt of a PR without a closure Derek should apply a label, such as `no-issue`, `unlinked`, `no-closure` and post a comment to provide the user with the information required to help them update the description.

## Steps to Reproduce (for bugs)
Not a bug

## Context
Enabling the ability for Derek to apply more of a project's contributing guide through contributor education & prompting.

## Your Environment

N/A