Automate PRs to master

[edmundmiller]

On PRs to master there should be an action at automatically adds it to the https://github.com/orgs/nf-core/projects/39 and ideally randomly assign a maintainer.

As an aside, not sure how we'd check release candidates if we fixed the need for a PR to master. We could have a requirement to have a nf-core maintainer review a PR for code-quality etc for every PR then, but I don't know if there's too many PRs.