Add a way to disable the squash commits feature when merging

See comment below:

Is there a way to disable this on a project or system wide basis? We don't want our engineers to be able to do this as commit messages kick off a ton of automation at our company (like versioning releases) and clicking this by accident can cause bad side effects.

I was not able to find a way to disable this.

One of the big challenges we've had with gitlab right now is that the UI workflows are changing which makes a big impact on our developers' workflows and we don't seem to be able to customize them. New features are being pushed out without the ability for us to say "that doesn't add value for us or it removes value" by toggling a setting to use or not use them.

This seems like something we could allow admins to turn on/off, like we do for approvers.