26226 installer issues

This PR primarily fixes two issues from #26226:
- A reported 500 error in the installer
- Visual issues with the installer

To test:
- Use the new phing target "build-no-install" to get the site ready, then run the installer manually at install.php.
As the 500 error appears to be a FastCGI problem, it is preferable to test it in that environment, and not using mod_php.