View customer contact counts for Service Desk issue

Context

• Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/1675

• Exclusive ~"EE Premium" feature only

Description

• View how many times a customer has emailed in for a Service Desk issue.
• View how many times an internal support staff user has responded.
• View counts in the issue UI somewhere.
• Sort by these counts in the issues list.