Curved lines at top of pipeline graph

#### What does this MR do?
Adds curved lines to second-child of each column
## Are there points in the code the reviewer needs to double check?

#### Why was this MR needed?
To match the mockup

#### What are the relevant issue numbers?
Closes #21126 

#### Screenshots (if relevant)
![Screen_Shot_2016-08-26_at_9.31.49_AM](/uploads/318d3df4ce909daa0ef8ea8d989211c4/Screen_Shot_2016-08-26_at_9.31.49_AM.png)

See merge request !6033