Implement a merge queue for squiddy

[elenatanasoiu]

As more people join FutureLearn it's becoming more important to have visibility on the order of deployments. At the moment a merge will immediately trigger a deployment.

We should consider instead putting a merge request in a queue, similar to the way Shopify's merge-bot works. 1 2

[elenatanasoiu]

Another solution was using https://github.com/bors-ng/bors-ng which can implement a merge queue.