Extend pipeline graph scroll area to full width

What does this MR do?

Part of #54331 (moved) (Updating pipeline graph looks)

On screens wide enough, when the window is wider than the fixed width container above the pipeline graph, extend the scrollable area of the pipeline graph out to the edges of the window. Set the default scroll position so that the left edges of the fixed width container and the pipeline graph line up.

before	after
Screen_Recording_2019-07-26_at_12.48.44	Screen_Recording_2019-07-26_at_12.47.20

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14870

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers