#93 use the WebDavHelper directly

The WebDavHelper is used directly and a server action for this task is
not needed anymore.
 #93 forbid the saving to the content field

The imported  files for the representations must not be saved inside the
content field, but they should be always uploaded to the WebDav.
Merge branch 'bug/93-corrections-in-BasicImportDocumentVisualPanel' into dev