Improve GitLab project CI Danger to better handle capitalization rule and not prevent contributions

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Motivation

Not being able to address my mistake of forgetting capitalization in commit message from GitLab UI is preventing me from getting my contributions merged. I can't imagine I'm the only person.

Background

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21527 one of the rules added is that all commit message subject lines must start with a capital letter. I personally disagree that the capitalization even matters but I can live with it. My issue however is how complex it is to address if you forget to capitalize in the first place:

• I do most of my trivial commits from GitLab UI on a computer or mobile device without ability to install Git
• I often forget to capitalize commit message
• My MR fails CI pipeline and I have no way to fix CI pipeline from GitLab UI as I cannot do history rewrite from GitLab UI
• Selecting squash commits and giving MR title a commit message with capitalization seems to have no effect on fixing pipeline

Solutions

• Improve Danger to detect if squash commits is checked in MR and MR title has acceptable message
• Provide squash commits button from UI that runs on demand (not just when merging MR)
• Provide method to do history rewriting from GitLab UI (e.g. edit commit message https://gitlab.com/gitlab-org/gitlab-ce/issues/32315 https://gitlab.com/gitlab-org/gitlab-ce/issues/57719)

/cc @rpaik @dplanella @meks @ddavison @marin