previewer: PDFtk previewer

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

* Allow annotating any page.

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

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

* Add infrastructure for adding targeted notes on records (e.g., on pages,
figures).

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

* Allow adding record notes (from annotations module) via plain comments.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>
annotations: JSON-LD (WIP)