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 

## Remaining tasks

* [x] Preserve `·` and ` `
* [x] Use XHTML 1.0
* [ ] Use the same layout (`app/views/layouts/notify.html.haml`)
* [ ] Digest or not (assets or public)
* [x] A similar email for succeeded pipeline
* [x] 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?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] `PipelinesEmailService`
  - [x] `SendPipelineNotificationService`

See merge request !6019