Resolve "Epic dates from milestones don't work when using /epic in a new issue"

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ee/issues/11750

When creating a new issue and adding an epic using a command in the description, if we are also adding a milestone and the epic start and due dates are not set to fixed these dates will not be updated.

We are currently updating these dates (if the issue's milestone changes and the epic exists) here: Issues::UpdateService. This MR adds this check to Issues::CreateService to cover the case where issues are created using commands.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers