Disable the merge train button while squash is selected

Problem

This is a quick UX improvement before we've done #13001 (closed). Here is a quote from gitlab-com/www-gitlab-com#5177 (comment 234737212)

I'm not sure if this is the right issue for this comment.

removed this merge request from the merge train because merge train does not support squash merge

1. Generally for good UX one shouldn't allow an action to be performed if it will lead to an error. We shouldn't let someone do an operation, send it into the queue, and then have them not find out until after it hits its place in the queue to tell them it can't be done. We should be doing that check before it gets placed into the queue.
2. What is the reason squash merges aren't allowed? Is it a technical limitation? Squashes are commonplace where I came from before GitLab. I feel like that would be a dealbreaker for some customers when it comes to this feature.

Propsal

disable the merge train button while squash is selected