Hierarchy level migration

[dorthrithil]

Now that we have flexible hierarchy levels, migrating between those would be nice. I actually already have an instance where this would be needed.

Migrating to less levels: Here we can create a temporary sector / area and move all lines and topo images there before skipping the hierarchy level (needs #439). Then the admin needs to manually move the lines and topo images to their actual destination.

Migrating to more levels: Here we can straight away use the placeholder areas & sectors. Then move the stuff with the feature coming in #439.

Blocked by #439

[dorthrithil]

@BlobbyBob Any thought on this? Would this solve it?

[BlobbyBob]

I think this is a decent idea, but it feels like there are many scenarios to consider. We should create a structured overview beforehand, so we do not end up with a solution fixing only half of the cases.

If manual operations are involved, we need some batched move operations, since no one wants to move hundreds of lines by hand :D