Remove usages of `gl-xs-flex-direction-column` CSS util

What does this MR do and why?

Remove usages of gl-xs-flex-direction-column CSS util

This replaces usages of the desktop-first gl-xs-flex-direction-column CSS util with mobile-first alternatives.

Screenshots or screen recordings

There is no visual change as we are essentially reproducing the same layouts but with mobile-first utils instead of desktop-first ones. Here are a few screenshots for reference.

Subject	Mobile	Desktop
Abuse report header
Abuse report content
Abuse report activity
Invite modal
File browser header

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.