Allow disabling "Squash and merge" for a project

Description

This feature is super-useful and we use it in a lot of our repositories. However, we have at least 1 repository where we want to be very careful about maintaining history between a pair of branches, when we merge across from one to the other we have to keep the exact same commits to keep the history working correctly. Since we use this feature in other repositories it's far too tempting for our developers to press it all the time, which results in a new commit being created diverging the branches histories.

Proposal

Add an option in the project settings that can disable and hide this checkbox for all merge requests in the project.