This fixes missing events when milestone or assignees are set
together with setting the epic on issue creation. This was
happening due to epic_issues relationship being saved before issue
changes were handled, which also triggered an issue.save which
would then lose issue changes. This moves the save of epic_issue
relationship to happen at issue.save time.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61746