comments: reviews_base template;

* Moves reviews.html to reviews_base.html; this is only a move commit and it is
broken, use child.

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

* Re-adds reviews template extending from reviews_base.html.

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