Add new quick action to squash commits

Proposal

This is pretty tied to GitLab's workflow, but it might benefit others.

Sometimes, the squash option needs to be enabled, otherwise Danger will fail if there's an applied suggestion. Right now, I have to edit the MR and check the squash checkbox. It would be nice just apply the quick action and then merge the MR.

Similarly, there should be a quick action for removing the branch when the MR is merged #15888 (closed).