Add a new pipeline email service

What does this MR do?

Add a new pipeline email service

What are the relevant issue numbers?

Closes #3976 (closed)

Remaining tasks

• Preserve · and  
• Use XHTML 1.0
• Use the same layout (app/views/layouts/notify.html.haml)
• Digest or not (assets or public)
• A similar email for succeeded pipeline
• Plain text versions for both emails

Screenshots (if relevant)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6019#note_16594345

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • PipelinesEmailService
  • SendPipelineNotificationService