Add tooltip component

The tooltip component was omitted when !34 (merged) was implemented.

But it will be the proper way to implement a number of existing and new tooltips that require dynamic content for example:

• https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/assets/javascripts/sidebar/components/sidebar_item_epic.vue#L57
• https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/assets/javascripts/vue_shared/components/sidebar/collapsed_calendar_icon.vue#L41
• https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/assets/javascripts/epics/sidebar/components/sidebar_date_picker.vue#L280