Adds workflow file to check for labels on PRs

[daslerr]

What does this change?

This PR adds a file (check-pr-label.yml) that creates a workflow for checking for release category labels.

Common Theme?

This PR doesn't change Common itself, but it adds a workflow that blocks PRs to common-theme if they do have have one of the release category labels outlined in releases.yml

• Yes

Issue number: #9

Checklist

• I have read the contributing guidelines
• I have checked my spelling and grammar with an automated tool
• I have previewed my changes to check the markdown renders as I intend
• I have run my code to check it works

Who needs to know about this?

@reshamas @SallyMcGrath @illicitonion

[daslerr]

Well, it blocked itself because it didn't have a label. So I guess it works.

[daslerr]

After it blocked itself because it had no label, I labeled it and re-ran the check, but it still failed. So I had to close and reopen the PR to get it to pass. According to the GitHub docs on writing workflows, by default a workflow only runs when a PRs activity type is 'opened', 'reopened', or 'synchronize', so I added some activity types to the yml to cover labeling, unlabeling, and editing as well. There are many others I did not add.

[daslerr]

And after I merged Reshama's PR #10 requiring semver labels, it triggered a check automatically on this PR that failed. When I added the semver label, the check triggered again and passed. So I think we did it. Hooray!

[reshamas]

@daslerr It looks like it is working!