Commit which adds a PDF Pool for managing PDF files in the Quiz Editor.
This fixes problems that are described in issue #27 (closed). The global idea of this fix is that PDFs are no longer passed through the components state, rather, the quiz editor passes an array of PDF IDs that are loaded by the new PDF Pool. Once the ID of a needed PDF is in there, the components that need this PDF will rerender. This fixes the problem of missing thumbnails and linter issues.