Add option to auto-rebase by default (and opt-out instead)

[llucax]

The opt-in filter label is useful for projects that need a fine grained selection of which PRs to auto-rebase (and in particular where it shouldn't be the default). In projects where auto-rebase should be the norm (where for example the PR branch is required to be up-to-date and no merge commit is desired for the update, as github does with the "Update" button), having to set the label for everything is annoying, redundant and noisy.

It would be awesome if there was a configuration to enabling rebasing by default, and even better if there was an option to select a (configurable?) label to opt-out. In this case it's probably a good idea to also automatically opt-out "Draft" PRs (as this shouldn't mess with WIP PRs).

[biancarosa]

Onboard with that. I'd also love path filtering when it comes to that, if I have a monorepo and would like to restrict opt-in to a certain path, that'd be extra cool.