View and sort how long Service Desk issues open

Context

• Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/1675
• Exclusive ~"EE Premium" feature only

Description

• View how long a Service Desk issue has been open, using closed_at - created_at, c.f. https://gitlab.com/gitlab-org/gitlab-ce/issues/27212.
• Some UI in the issue to indicate this information, integrated with due date / SLA (https://gitlab.com/gitlab-org/gitlab-ee/issues/2031) and time tracking perhaps.
• Sort by how long issues have been open in the issues list.