Resolve "Pipelines for merge request"

## What does this MR do?
Adds `Pipelines` tab in merge request view

## What are the relevant issue numbers?
Closes #18681 

## Screenshots (if relevant)
![Screen_Shot_2016-08-16_at_3.22.41_PM](/uploads/c04febab3765b1fac2bf3bbfb9882f9f/Screen_Shot_2016-08-16_at_3.22.41_PM.png)

See merge request !5485