Support relative paths for index.html loads

[joshuaboniface]

Allows SUI to work at arbitrary path names rather than relying on fixed root-level paths.

[magikmw]

@joshuaboniface Let me know if I got this right:

• right now sui can only run on paths like sub.example.com/
• your change makes it possible to run on paths like sub.example.com/directory/index.html

[joshuaboniface]

@magikmw Yep, that's right! As a test, I put SUI at myserver.tld/sui/ under a manual web root (no Docker) and these assets were all trying to be loaded out of myserver.tld/. Now they'll load out of whatever directory index.html is in.

[magikmw]

@joshuaboniface Thanks! I've merged the change with my fork.

[jeroenpardon]

Thanks