Preparations for beta test

[capital-G]

• Write a tutorial (add tutorial #634 )
• Make start/about/end text not empty
• Changing of audio file for an audio is updated in DB but not in frontend.
• Allow more audio formats for upload (only flac/wav right now)
• Right now the content of the script cells are loaded on entering the node - maybe we can reload it from the database after a script cell has been executed so the updates are faster?
• Make selected playback style more obvious
  
• How to trigger GPS data - add disclaimer when it works?
• Add disclaimer that mic does not work when screen is locked
• Document (and improve?) hardcoded streaming variables as triggers - for example ready, start, about, end etc