Disable Service Desk activation if issue tracker is disabled

...ts/settings_service_desk/components/service_desk_root.vue

...settings_service_desk/components/service_desk_setting.vue

...ssets/javascripts/projects/settings_service_desk/index.js

app/views/projects/_service_desk_settings.html.haml

locale/gitlab.pot

...ettings_service_desk/components/service_desk_root_spec.js

...ings_service_desk/components/service_desk_setting_spec.js

...ce_desk/components/service_desk_template_dropdown_spec.js