Update file row styling

What does this MR do?

This MR cleans file_row component, so it's styling does not more intervene with other components. This is important as new release of gitlab-ui will introduce change of internal class names for gl-loading-icon, which will break styling here (see gitlab-ui!690 (merged) for details)

We're changing styles so file_row just controls styles of file_icon component, without knowing anything about its implementation details

Screenshot

This also fixes a preexisting issue

Before	After

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