New course element "Document"

A new course element is implemented that allows embedding of a document. The document can be 

• read only
• read and write
• downloadable

depending on the available document editors and the configuration settings. 

Documents can by any kind of files, however the main usecase is meant for office documents, PDF and maybe some images. 

The source of the documents can be either

• an already existing document learning resource
• a file from the course folder
• a newly uploaded file

Uploaded files can be converted to learning resources for reuse in other courses. Existing learning resources can serve as a template by making local copies from them.