Change to dynamic width (Fixes gitlab-org/gitlab-ce#66195)

What does this MR do?

This MR fixes gitlab-org/gitlab-ce#66195. A SCSS variable introducing static width into a container with vertical overflow is creating an unnecessary horizontal scroll.

No testing should be needed beyond additional cross-browser testing. Fix appears functional in Safari, Chrome, and Firefox.

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

Performance and Testing

Shouldn't pose any risk; if anything, negligibly shears down compilation time? I would cross-browser test this in IE/Edge, but it appears to work in Safari, Chrome, and Firefox.

• Tested in all supported browsers