Do not update `updated_at` on an epic when reordering it.

Description

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/44949#note_185903054

This case is similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/44949, it's not expected behaviour to update timestamps when a resource only changes its relative position.

Current Behaviour

When reordering child epics, timestamps are updated

Output