annotations: record document annotations

* 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: annotations integration

* Integrates the record document annotations with the Invenio commenting
  facilities and PDFtk previewer.

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

* Add JSON-LD publishing utilities for annotations and RESTful API helpers.

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