Use SitOnIt-CI to enforce minimum lifetime for Pull Requests

I've noticed it's common in this repository to wait for a while before merging a PR that has been rapidly reviewed in order to give other maintainers a chance to comment themselves. At this time, this is more of an unwritten courtesy; what about making it policy?

[SitOnIt-CI](https://github.com/apps/sitonit-ci) enforces just this, and is configurable per-repository.

I would suggest 24 hours as a required wait time; this is the default and requires no configuration.

As with any CI status service, any user with admin privileges on the repository may install the app or override it to merge anyway.

I would love to see some discussion about the pros/cons of enforcing wait times.