Automatically assign colors for remote multi channel dataset

[frcroth]

Multi channel datasets can be imported whereby every channel becomes a datalayer (#6578). When viewing the dataset, these are all white. When multiple layers are added this way, they should have different colors. For ngff multi-channel dataset, the omero/channels attribute may already include colors, which can be used here (eg. https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0101A/13457227.zarr)

Context

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