Allow Service Desk to be configured by project admin

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

Current

• Currently Service Desk can only be toggled on/off by the global instance admin.
• A project admin cannot toggle on/off the feature per project.
• A project admin can see the feature toggled on/off, and can see the email address.

Proposal

• Allow a project admin to toggle the feature on / off.
• This would allow usage on GitLab.com.