BigBlueButton plugin: pre-upload slides

[ywarnier]

The latest API of BigBlueButton allows one to pre-upload documents to the videoconference room. This is great (we asked for it) and we should implement it in the plugin, as follows:

• in the conference tool in a course (only when in a course), if the user is the teacher, offer a folded section called "Pre-upload documents", loading the list of documents from the documents tool. Only load this list in AJAX.
• the list should only show PDFs, PPTXs, ODPs and images as a "flat" list (no directories structure), and offer a checkbox next to each of them
• when starting the videoconference room, use the list to pre-upload the documents using the corresponding API call (see http://docs.bigbluebutton.org/dev/api.html#pre-upload-slides)

[christianbeeznest]

It is implemented with this commit christianbeeznest@d6cf215

Thanks for confirmation.