Squash commit template

Problem to Solve

Currently there is no way to enforce a standard squash commit message on merge requests. This means the squash message needs to be crafted by the maintainer upon merge, or the default squash message from GitLab is used.

Proposal

With the addition of a merge commit template in !64437 (merged), this functionality should be extended to support the squash commit. This would provide a standard way to configure both types of commit messages for organizations.