Prevent WK worker job execution for remote datasets

[hotzenklotz]

Detailed Description

Every once in a while some people try to execute a WK worker job on a remote dataset. This does not work and will result in the worker crashing.

We should check for these cases before executing the worker job and present the users with a good error message. Perhaps this is a backend issue or perhaps we adapt the various modals and components in the frontend.

• Specific to long-running jobs (set jobsEnabled=true in application.conf)
• Specific to webknossos.org (set isDemoInstance=true in application.conf)