web/.htaccess: auto-add trailing / if not external redirect: added test if `%{REQUEST_FILENAME} -d` in addition to checking %{LA-F:REQUEST_FILENAME}, because there are some cases (e.g. subdirs that are symlinked outside the DOCUMENT_ROOT) where LA-F:REQUEST_FILENAME for some reason appends /home to the REQUEST_FILENAME, but the current REQUEST_FILENAME is fortunately already a dir and can be checked directly