roughly revert cb7e906f01f9a (change all users...)

instead of doing a real revert, this uses the script described in
`WikiGardening/index.mdwn` in reverse; in other words, this commit is
the application of

for user in $(find ./Benutzer -type d -maxdepth 1 -mindepth 1 -exec \
basename {} \;); do find ./ -type f -name '*index.mdwn' -exec sed -i \
-e 's:\['$user'\](/Benutzer/'$user'):\[\['$user'\]\]:g' {} +; done

due to the new userdir setting, user names can be used everywhere and
will work as if relative to Benutzer/; also, now the duplications of the
link text and target, which stem from the moinmoin conversion, are
removed.