Add co_authored_by to merge commit templates

What does this MR do and why?

This merge requests add support for %{co_authored_by} variable in a merge commit templates. This solves #20421 (closed).

Variable expands to list of commit authors in a form:

Co-authored-by: Zane Doe <zdoe@example.com>
Co-authored-by: Alex Garcia <agarcia@example.com>

This format is often used by various tools to add more authors to a single commit, and would be most useful in a squash commit template. The format is already somewhat supported by gitlab, as the user mentioned that way is linked to in a commit view (see screenshots section).

A lot of complexity was added by the fact that the main commit author shouldn't be mentioned as a co-author. For merge commit, that means we should skip merging user (for non-merged MR we assume currently logged in user), while for squash commit we should skip MR author. We could simplify things a bit and future-proof this if we decided to ignore that and always include commit author as a co-author as well, so let me know if you'd rather have the simpler, but a bit less user-friendly version.

Screenshots or screen recordings

Docs:

The format is already somewhat supported by gitlab, as the user mentioned that way is linked to (it would just be a mailto link if no account would be tied to given email address):

How to set up and validate locally

1. Go to any project -> settings -> general.
2. Expand Merge Requests section.
3. Fill in Merge commit message template or Squash commit message template with new variable %{co_authored_by}.
4. Commit a new branch with couple commits created by different authors (with different emails).
5. Create a new MR from this branch.
6. Check that the default message in MR widget renders co authors correctly.

MR acceptance checklist

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