Add MR links to Pipeline view
### Problem to solve
When I check for pipeline results in pipeline view there is no link to related MRs on the page. It's not easy to find a MR this pipeline was triggered for.
<!-- What problem do we solve? -->
### Target audience
All pipeline users.
### Further details
Example page:
https://gitlab.com/gitlab-org/gitlab-ce/pipelines/48689904

### Proposal
<!-- How are we going to solve the problem? Try to include the user journey! https://about.gitlab.com/handbook/journeys/#user-journey -->
- Add a separate related merge request info line in the widget on the pipeline view.
- It includes merge request information similar to the commit view widget
- It is only shown for branch pipelines (not merge request pipelines either attached or detached)
If no related merge requests are found:
- `No related merge requests found`

If a single merge request is found:
- `1 related merge request: !1 Merge request title`

If multiple merge requests are found:
- `2 related merge requests: !2 Merge request title, !1 Merge request title`

__Note:__ Merge request titles should not be italicized. This is a small error in the mockups.
### Documentation
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### What does success look like, and how can we measure that?
User takes one click less to get to MR page from pipeline view page.
**AC1**:
* Given Pipeline 1 was triggered for a commit in MR "A" and in MR "B"
* When I'm on Pipeline 1 view page
* Then I should see a link to MR "A"
* Then I should see a link to MR "B"
### Links / references
issue