Fix file header text wrapping on long paths on Bitbucket

Created by: marekweb

Fix #11110

Summary of attempts to solve this issue:

• Didn't find a way to keep using display flex while making the file path wrap
• No clear solution to wrap both file path and toolbar buttons: because they're on the same line, the desired wrapping behavior isn't obvious
• Simplest apparent solution: remove flex and use aui-buttons class which already exists and is used on Bitbucket toolbar buttons.

Limitations of this solution:

• Toolbar buttons don't wrap if there's an excessive number of them. The file path (sharing space with the buttons) will wrap instead.