Add PR gate to ensure checklist has been completed

[richard-cox]

Summary

Add PR gate to ensure checklist has been created

Occurred changes and/or fixed issues

• There's been PRs that have merged without the required checklist being completed (for instance missing milestones, no reviewers, missing template, dark mode not tested, etc)
• This PR will ensure all PRs have every checkbox in the pr description checked
• This does not ensure all required checklists entries are checked, we can update in future if that gap is being abused
• Gate will check on pr creation and description edit

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes

[nwmac]

Looks good to me.

Maybe we should beef this up later and check that the checklist is intact - otherwise I can just delete the checklist and it will pass!