Smaller min-width for MR pipeline table

## What does this MR do?
Nests class under `.tab-pane` to make sure buttons don't get cut off

## Screenshots (if relevant)
![Screen_Shot_2016-10-14_at_11.00.02_AM](/uploads/6e19e0375721f2cb58223c2e7ed89308/Screen_Shot_2016-10-14_at_11.00.02_AM.png)

## What are the relevant issue numbers?
Closes #23346

See merge request !6893