fix url for sub-path hosting

[lovelylain]

1. Add a slash to the nav-bar index link: http://xx.xx/xx to http://xx.xx/xx/
2. Use relative path in tandoor.min.css and tandoor_dark.min.css.

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[vabene1111]

thanks for your PR. I am currently reworking the frontend which will make this change obsoloete (see #2976 )

Since this has the potential to break something I will not merge this PR. Feel free to use it for your own benefit and let me know if the new version has any issues like this as well.