[etwist] fix handling of multiple files per field html5 permits multiple files uploads, which can be expressed as:: <input type='file' multiple='multiple' /> This changeset avoids previous crash. Nothing is changed when a single file is uploaded (backward compat is thus preserved). When multiple files are uploaded for a single html input tag, the corresponding web request form key receives a list of tuples like [('filename-1', IStream1), ('filename-2', IStream2), ...]. closes #2847207.

What's new in CubicWeb 4.0?
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Deprecated Code Drops
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* The ldapuser source has been dropped. ldapfeed is the only official source
  remaining for ldap.