Upgrade to Nextcloud Hub 19 - Done

Todos:

• We usually wait for the first bugfix release 19.0.1 (skipped*)
• Update Talk App 9.0.1 (done)
• Update Deck App 1.0.3 (done)
• Update Calendar App 2.0.3 (done)
• Update Contacts App 3.0.3 (done)
• Update all customers clouds to the last version of Nextcloud 18 (done)
• Adjustment of the autoinstallation (done)
  
• New customers get Nextcloud 19.0.0 installed (done)
• Eat your own Dogfood. Update of our own clouds (done)
• Test the migration of existing customers (done)
• We noticed a bug regarding Collabora Online (Github) (we have a workaround)
  
• Prepare info page regarding the upgrade to Nextcloud Hub (done)
• Customers get updated with the new space, see the new product page (done)
• Migraition of all customers / send information email (done)
  

* Nextcloud version 19 is stable enough and all our apps are working.