System note for setting milestone at creation for issues and merge requests

• Follow up https://gitlab.com/gitlab-org/gitlab-ce/issues/46479.
• When creating an issue or merge request, and you've set a milestone, add a system note.
• Let the system note be @victorwu changed milestone to 11.1 · at creation
• Track the event per the technical changes in https://gitlab.com/gitlab-org/gitlab-ce/issues/46479.

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