v9: bugfix 11228 Blocklist stylesheet path includes wwwroot

[Zeegaan]

Fixes : #11228

Notes

• Updated blockconfiguration.overlay.controller to use the staticFiles tree picker

How to test

• Run a clean umbraco install
• Create an element type with a textstring property
• Make a doc type with template and allow as root
• Make an empty stylesheet called "myStylesheet"
• Add a blocklist editor to your root doc type, and pick the stylesheet from the CSS folder
• Go and make some content, and when you type something in the textstring and the CSS pops up, it should now call the right URL "localhost/css/myStylesheet.css

[elit0451]

Tested and works like a charm 💪 Will merge once the tests pass 🔜