Market service desk in EES

• Service desk is an EEP feature. https://gitlab.com/gitlab-org/gitlab-ee/issues/149
• Market service desk inside EES.
• In EEP, service desk is activated by checking a checkbox in the project settings.
• In EES, the checkbox should be disabled, with a link to the docs page and/or the product / feature page, explaining that it is an EEP feature.
• It's not absolutely clear that this would be a great feature and would convert a lot of people. But the effort is fairly low and thus a good tradeoff.