Migration to Nextcloud 17 - Done

We already working on the upgrade to Nextcloud 17. The progress can be followed here:

• Wait for the first bugfix release 17.0.1 (done)
• Adjustment of the autoinstallation (done)
• New customers get Nextcloud 17 installed (done)
• Update all customers clouds to the last version of Nextcloud 16 (in progress)
• Test the migration of existing customers (done)
  
• Eat your own Dogfood. Update of our own clouds (done)
  
• Migraition of all customers (done)
  

(The todos are sorted by priority.)