Feature: Support CLA checks in merge queues

[brian-smith-tcril]

GitHub has added a feature called "merge queues" (docs) that feel like a very good solution to some of the race condition issues with automatic PRs I've encountered on https://github.com/openedx/openedx-translations/

When adding a PR to a merge queue, it will try to get the CLA status, but the hook will never return anything

I have added "Merge groups" to the events that trigger openedx-webhooks, but I have not started looking into what changes need to happen in code here to allow running the CLA check for PRs in a queue.

Acceptance Criteria:

• When a PR is added to a merge queue, the CLA check runs and correctly reports the status.

[nedbat]

We can debug why the bot gets stuck in this situation. I have no experience with merge queues, so I'm not sure how they affect things.