Courseware 2.1

• add new block: AudioBlock
• add new block: DownloadBlock
• imporved iFrameBlock with user specific token
• prevent navigation on change for all blocks
• TestBlock: practise has to be submitted but not corrected to continue
• VideoBlock: fixing openCast URLs
• cleaning author views
• switch from png icons to svg icons
• adding icon to studip course overview (counting new blocks, sections and chapters)
• adding plugin preview image