Track changes to Epic start and finish dates in Epic system notes

Summary

With %10.7, Epics will have full commenting support similar to Issues (see https://gitlab.com/gitlab-org/gitlab-ee/issues/3889). Currently, the feature is in development, and system notes that appear along side comments do not show changes made to start and finish dates from the sidebar.

Proposal

System notes should show everything that can be updated about an Epic.