Use Alias for /sitename when using legacy URLs
An attempt to solve INC2594709 and hopefully other situations where CGI scripts stopped working after migration because of changes in URLs.
This still needs to be tested.
We lose the redirection /something -> /sitename/something. That redirection was intended to keep bookmarks working if a site was reconfigured from new scheme -> old scheme (e.g. site owner found problems a while after the site was initially migrated with the new scheme). We do not have that situation anymore because the migration process eventually enabled the legacy URLs by default for all sites to minimize problems, so now the only transition we need to support is old scheme -> new scheme.
NB: we'll have to adjust current integration test that verifies redirection https://sitename.webtest.cern.ch/test -> https://sitename.webtest.cern.ch/sitename/test to verify redirection https://sitename.webtest.cern.ch/ -> https://sitename.webtest.cern.ch/sitename/ instead