Tasks queue improvements

Requirements:

• some tasks need to be chained (eg image compression -> thumbnail generation)
• some tasks may need to be done as fast as possible (eg single line polygon recalculation which is done live for now)
• the user should be able to see the number of tasks still in queue (and maybe a warning if the server is currently on heavy workload)
• identify tasks that could be done easily on another server, to unburden the database/gpu server, for scaling
• load balancing for cpu, gpu and memory, one user shouldn't use all resources if other users are queuing tasks
• keep enough resources for db and web server
• better feedback on tasks that did not reached the worker yet
• when deleting a document or parts, cancel related tasks/processes.

Technical considerations:

• check other frameworks than celery, mostly to see if load balancing is easier to do
• could spawn a dedicated queue by user with --autoscale to simplify dividing workload to all concurrent users?