previews: refactoring

* Changes preview view function arguments, passess request arguments;
* Sends both filename and file from preview to extensions;
* Fixes PEP8;

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
bibauthorid: syntax error fix

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
previews: PDF preview move

* Moves the PDF preview demo from comments to previews.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
utils: unicode fix

* Accepts unicode as well as strings for escape_shell_arg();

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
previews: PDF (WIP)
translations:fixes for string messages
previews: pdf viewer rename

* Renames pdf to pdftk.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>