Webapp: React on status change callback

Created by: ghost

on status update (i.e. when callback received)

• use url from payload to request current status for the respective format (depends on #257 (closed))
• update own status in database
  • if changed: notify user (within web UI)
• when status == finished
• retrieve file for respective format (use URL from status request answer) (retry if necessary?)
• request file deletion on other side if retrieval was successful (use URL from status request answer) (tracked by #279 (closed) , #139 (closed) and #289 (closed) )
• when all formats of order have a final status (finished or error)
  • notify user (by email)