Noticed there wasn't any tooltip on the clipboard button so I moved to
using `data-clipboard-text` which gets handled by `copy_to_clipboard.js`

 - Prerender data-* attributes for endpoint, enabled, incoming_email to
   hydrate Vue component
 - Use actual /group/project/service_desk endpoint GET/PUT

Address @filipa review

 - Accessible icons
 - nits

@hazelyang metrics,
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1519#note_26863100

(including spam)

... automatically when it goes from false to true

unless explicitly unsubscribed

(also if service_desk_enabled is off)