comments: PDF viewer addition

* Adds a simple PDF viewer, available in the comments/notes page;
* PDF viewer is integrated with the commenting feature: comments can be added
on the previewed page, and notes can be viewed per page.
* Requires CFG_PATH_PDFTK and CFG_PATH_IMAGE_MAGICK_CONVERT.
* Refactors comments JavaScript;
* Feature can be disabled via CFG_COMMENTS_PREVIEW_ENABLED.

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