Send notifications for epic status change

What does this MR do?

It adds notifications for closing and reopening epics.

• it adds close_epic and reopen_epic methods in NotificationService, both call epic_status_change_email and use one template (epic_status_changed_email)

Custom notification settings are not included in the scope of this MR - follow up issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/8281

What are the relevant issue numbers?

Closes #7272 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?