Pull request automation

[aexvir]

Similar to GitLab's merge trains, but without the hassle of having to use merge request pipelines nor merge commits

Initially, this should be about merging commits automatically as long as checks/CI is passing. Internally we'll use some queue mechanism where we'll stack commits to be merged there. For each MR in the queue, we'll take it, rebase it on master, wait for checks and merge if they pass.

In the future we might add more configurability to it, like commands in the PR to alter the order, multiple strategies for when one MR in the queue can't be rebased, possibility of enabling/disabling this functionality on a per PR bases (with labels, or command or whatever)

This should work in both GitLab and GitHub