Resolve "No space between buttons on Files page"

What does this MR do?

Adds a .download-button class to the project download button and added 10px of margin to this button within the .tree-controls.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Whitespace is gr8!

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23380 (closed)