Instead of further handling requests to `public` which causes issues
like go-gitea#11088, immediately
terminate requests to directories `js`, `css`, `fomantic` if no file is
found which is checked against a hardcoded list. Maybe there is a way to
retrieve the top-level entries below `public` in a dynamic fashion.

I also added `fomantic` to the reserved usernames and sorted the list.

Fixes: go-gitea#11088