Add support for multiple Guacamole instances

[MoritzWeber0]

We should allow different instances of Guacamole properly:

• Just scaling to 2 instances doesn't work (Guacamole produced "Forbidden" errors), needs further investigation
• Add Guacamole instance management in the frontend, each session is assigned to exactly one Guacamole instance, no switching in between instances
  • The default Guacamole instance should be auto-created so that there is no need for migration (it just continues to work)
• Authentication might be more difficult when they run on different domains (currently, we inject the auth creds in local storage, which only works on the same domain)
  • Development of own auth-plugin might be needed