Add option to have mergify config file saved in the UI

[enlivn]

What

Give users the option to either save the .mergify.yml file in the default repo branch (status quo) or just have it saved in the UI (and you can save it on your servers so it preserves its state)

Why

This helps iterate on mergify much faster because I do not have to keep making updates to the default branch to test out changes (yes, I'm aware I can check the configuration in the UI against a PR but I can't see the result of an action eg). Our default branch gets a lot of traffic and any time I commit a mergify change, everyone on the team who is in the queue has to wait for another round of CI since we require PRs to be up to date with the base branch. I imagine this is the case for a lot of users out there.