assets: cleancss url rebasing

* Adds cleancss assets filter with support for rebasing relative URLs
  against COLLECT_STATIC_ROOT.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
previewer: Mozilla pdf.js viewer component

* Adds pdfjs viewer component from Mozilla pdf.js as a single commit.
  This commit is needed since the viewer part of pdf.js cannot be
  installed via bower.

* NOTE: All source code in commit is copyrighted by Mozilla
  Foundation and licensed under Apache License Version 2.0.

Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
previewer: initial pdf.js integration

* Integrates pdf.js support in Invenio including customization of
  default pdf.js viewer.

* Fixes problem with pdftk bundle naming collision.

* Fixes problem with jQuery loading in records module.

Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
previewer: zip preview and styling fixes

* Fixes issue with ZIP files with no content.

* Fixes minor styling issues.

* Changes icons from Bootstrap to Font Awesome.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
docs: sphinx target not found for ExternalTool fix

* Adds new alias for `webassets.filter.ExternalTool`.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>