Danger does not lint the squashed commit message

So @akohlbecker did a great job discovering that we are actually not linting the squashed commit message at the moment. It's described in details in !18079 (closed)

I also verified that !17964 (merged) was squashed and merged and the commit message was just the merge request title, which we are NOT linting at the moment.

I think we should lint the title when squashing is enabled to make sure it won't slip through easily.

/cc @mayra-cabrera