Block PR merger on "do not merge" label

[rainest]

Problem
The "do not merge" label doesn't enforce itself.

Description
In #1522, I did not see the "do not merge label" because I was looking down at the end of the PR to see recent commits and CI status. Those looked good, so following Shane's merge of next into the PR branch, I thought I'd be proactive and merge the PR after CI went green, to avoid it getting out of sync with next again.

The consequences here were pretty minor (some dirty commit history went into next), but it's easy to not scroll back up to the top of a long PR to not see the "do not merge" label. Automation could help avoid similar screwups, with possibly worse consequences, going forward.

Making PRs into drafts works for this, but we don't typically use that, or only use it early in a PR's life. The "do not merge" label works well if the author of a PR wants to mark something that they don't want others to merge on their behalf so they can clean history or whatever. https://github.com/dimagi/required-labels should handle this easily.

Acceptance

• If "do not merge" is present on a PR, Github will not allow you to merge that PR.

[rainest]

Hasn't really come up in practice since, so closing as "maybe revisit this in the future, but not prioritized for now"