Fix merge_after moving on each edit if server timezone is not UTC

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What does this MR do and why?

Fix merge_after moving on each edit if server timezone is not UTC

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Merge delay time updates every edit if server t... (#512345 - closed)

MR acceptance checklist

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How to set up and validate locally

1. Change GitLab timezone to something else than UTC (For example using Japan as in the issue). This has to be done in config/gitlab.yml. (This change will be reverted by the next gdk reconfigure)
2. Restart GitLab (gdk restart) to load the new timezone
3. Edit a MR and set a "Merge after" time.
4. Save the MR and open the edit page again.
5. The time should not have changed (if you test this without the changes of this MR, the time will shift by some hours, depending on your chosen timezone)