Create preview PRs using labels

[victorlin]

Description of proposed changes

Benefits:

1. Ease of use - straight from the PR page
2. Workflow runs will show on the PR checks¹
3. Downstream PRs will be automatically updated with new PR changes
4. Simplified logic in reusable workflow

¹ In the case that a preview label already exists and another preview
label is added, this results in the workflow for the label that already
exists to appear as "skipped" because it has been skipped on the latest
event trigger to avoid an unnecessary update.

Checklist

• Checks pass, though they don't test the changes here
• If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR no functional changes
• (to be done by a Nextstrain team member) Create preview PRs on downstream repositories.
• PR previews are triggered on label
• PR previews are auto-updated on new PR changes
• Workflow runs show under PR checks
• Dev docs updated

[jameshadfield]

I'm not the best person to give a thorough review of GitHub actions from a technical perspective, but the functionality looks great. I haven't seen this approach used before -- did you see it elsewhere or come up with it yourself?

[victorlin]

I haven't seen this approach used before -- did you see it elsewhere or come up with it yourself?

The idea popped in my head while working on #1765. I searched online for "trigger on label" which led me to a StackOverflow example, and I took it from there.

[victorlin]

I'll merge this now since this is a change on the dev side that we can iterate on.

[tsibley]

Did anyone ensure that labels can only be added by us and not random other GitHub users? Because I'm pretty sure anyone can add labels… and that's not what we want here.

[victorlin]

That's a good concern, it hadn't crossed my mind. But I don't think anyone can add labels... I just tried on a random PR I had opened in bioconda-recipes and don't see a way to add a label other than tagging @BiocondaBot please add label which is something they had to strictly allow.

From GitHub docs:

Anyone with triage access to a repository can apply and dismiss labels.

[tsibley]

Yay, that's great. I'm not sure why I recalled being able to add labels to issues/PRs I've created in third-party repos…