Service desk integration

Service Desk Integration MR

• FE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1396
• BE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1508

Todo

• Update to actually server-side render whether service desk is enabled
• Use actual PUT /group/project/ { service_desk_enabled: true } returns { ..., service_desk_enabled: true, service_desk_incoming_email: 'foo@bar.com' }, https://gitlab.slack.com/archives/C440Z22MC/p1490838688303764
• Make new-reply and thank you HTML emails
  • Barebones reply email, https://gitlab.slack.com/archives/C440Z22MC/p1490635198533640
  • https://gitlab.slack.com/archives/C440Z22MC/p1490982356034328
  • Thank you email, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1508/diffs#note_26619624
• Show associated service_desk_reply_to email next to support bot username in issue (so responders know who they are emailing), https://gitlab.slack.com/archives/C440Z22MC/p1491245017034025

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cc @jneen @MadLittleMods @victorwu