Squash and merge in GitLab Core (CE)

Description

• Bring squash and merge (https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html) to GitLab Libre (formerly CE, see https://about.gitlab.com/products/) and GitLab.com Free (https://about.gitlab.com/gitlab-com/).

Original description

This issue is a request to bring the feature implemented in #4106 (closed) to Community Edition.

There are already many comments about this in #4106 (closed) but I felt it is better to have a separate issue for this.

I counted at least 65 "+1 for CE edition" from issue #4106 (closed). Please use the 👍 button here to show support for bringing it to CE.